The order where 了 appears in the sentence is important: when placed at the end of a sentence, it represents a change of state (it has nothing to do with tenses); when placed at the end of a verb, it represents a completed action. Using the examples in your question:
他们给我写信了 They wrote me a letter. (This is the first time I receive
a letter from them.)
I think Chinese textbooks should start their 了 sections with this:
了 is not about time.
了 is not about tense.
You are only concerned with 了 as an the aspect marker, aka completed action 了, or perfect aspect 了, so:
"昨天去商店" and "昨天去了商店" are both valid verb phrases. The second one explicitly states that the action was completed, whereas the first one ...
You have to remember, unlike in English, we do not have past tense for verbs in Chinese grammar. Both "is" and "was" is written as "是" in Chinese.
"是" in "她是好老师" could be "is" or "was". We really don't care which, because it is presumed readers can find out the sentence is in past or present tense by looking into the context.
Just to expand on Hugh’s answer a bit.
To understand what’s wrong with ‘我作天去商店.’ standing alone, we could translate it as ‘Yesterday I was going to the shop.’ Speaking English, if you said this and just stopped, the listener would think, well so what?
There are some verbs which are not used with 了 where a time phrase is enough to show past action. For ...
Yes, `one day' can certainly be used to refer to events in the past as well.
A better translation of "One day, the boy went for a walk" could be "有一天, 那个男孩外出散步", or less literary, "那个男孩出门走路". "去了走路" is not grammatical, "去走路了" and "走路去了" are grammatical but do not fit in here.
If the form 我看了一本书 is correct then what role is 了 playing here and why does it sound to me like past tense with a not-a-tense-marker indicating tense?
'了' in "我看了一本书" is a [aspect marker] indicating completed action.
我看一本书 = I read a book
In 我看了一本书, the action '看' is completed, which imply 看 is not in present tense but in past tense.
「我病了。」 can be a subtle expression which implies different meaning based its context. It can mean, but may not be limited to, these:
Present status that I'm not so well.
Past status that I was ill.
Completive sense that I've been ill for a while.
「病」 the word is a common one, which can imply from a minor ailment to a mortal blow. Again, this depends on the ...
You have a few options. As @NS.X. pointed out 一直 is a good one, but you can also use
一向, e.g.我一向很懒。I have always been very lazy.
向来, e.g. 她向来就是个书呆子。 She has always been a bookworm.
从来 is more likely used together with 不 or 没 conveying negation, of course that is not necessary.
A conjecture of mine: 一直 is often used with a certain beginning in the ...
If you used to be x and you're not any more you'd just use a construct like:
Of course, 很 doesn't necessarily mean really or very, but if you want to change it to 有点 that's cool too:
I would say, as a Chinese, that Chinese language doesn't express tenses as explicitly and clearly as English does in most cases. Your translation of the first sentence(我在北京住了六个月) can be taken in both ways, in which case we figure out the tense by context. Past tense would be a better guess, though.
To remove uncertainties, you can say:
我在北京住过六个月。I lived in ...
I learn that "过" is used for past actions, "会" = "will" (for future actions). So, can both 过 and 会 turn up in this sentence?
Your understandings of both characters are correct. But here in this sentence, the usage is a bit similar to "subjunctive mood" in English.
The sentence basically means: "After raining, the air would be much better." You can ...
Chinese language doesn't have tense. To indicate event happened in the past, you need to add time reference in the context or verb particle that indicate past verb
我的朋友今年在美国(住过) = My friend (had lived) in America this year
过 is a verb particle that indicates 'experienced' aspect of the verb
我的朋友今年(曾住)在美国 = My friend (had lived) in America ...
There's several explanations already, but IMO they are just expanding on your question and not really fixing it, the straight up answer and the most fluent way is this.
Q: 你怎么知道的？(How do you know that?)
A: 我从一本书上看的。(I read from a book.)
"从" translates to "from", "一本" is needed because you are specifying you read from A book and you know which book it is, ...
This is a frequent type of question, perhaps someone can ask a more general question that would allow a more generally useful answer. As a step toward that, I'll try to raise a couple of general points here.
First, past tense is not a vocabulary item in English. It is a type of verbal inflection. Chinese does not have this type of verbal inflection. Answers ...
[沒+ X(v)] is the negative form of [有+ X(v)] (replace 有 with 沒)
[沒殺人] (did not kill someone)
[有殺人]( have killed someone / did kill someone)
*Replace 有 with 沒
[沒有+ X(v)] is also the negative form of [有+X(v)] (add 沒 before 有)
[沒有殺人] (did not kill someone)
[有殺人]( have killed someone / did kill someone)
*Add 沒 before 有
（１）我在北京住过6个月。我在北京住了六个月 seems ok too
as regards word order if 在 follows 住：
In fact 在了 seems to be a familiar sequence，also one can find example sentences online with 住在了，iciba immediately produces ３ of them， here is the first： 当他年老的时候他住在了一个群岛上, 远离这里.
In case 了 is replaced by 过，it seems the corresponding word order has to ...
了 functions both as aspectual and modal particle, in the former case it occurs after the verb, in the latter at the end of the sentence. In images 1,3 the completed aspect of the action is referred to, in images 2,4 了 occurs at the end of the sentence to indicate a change（the emergence of a new situation, change in understanding, opinion,ideas, or action, ...
"打网球" is the fact; "今天早上" is the relative phrase that indicate what time it happened. Remove the relative phrase "今天早上" and write "我们打网球了", the sentence is still a complete sentence. (we still know you've played tennis, just don't know when)
[是] ([今天早上打网球) [的]
the entire phrase "今天早上打网球" is ...
以后 means "after". 过 is used for done actions. So the part before comma means: "After the rain has finished falling" (or, more in English: "After the rain").
The part after comma means "the air will become better". As far as I understand, you don't have troubles understanding why.
All in all, the sentence means "The air will clear up after the rain". As you ...
Question: When is the air fresher/cleaner?
After the rain
the air is/will be much better
1 is time, 2 is a statement
You could translate thus:
下过雨以后， After the rain has fallen,
空气会好很多。 the air is/will be much better
You might think 3 is past tense, but actually this has not yet happened, it's a projection in time, a 'when' in the future....
“了” is a special character usually meaning "finished" or "something happen in the future", we can summarize these points:
1) For a verb that can be persistant, “了” means “begin to do something immediately on spoken” （usually SVO+了）：
妈妈，我*做功课了*；做完功课后我出去散步。(Mon, I'll start homeworking, then I'll walk outside).
However, if SV了+O（Subject+Verb+了+Object），mostly ...
I think whatever you put 了 after verb or at the end of of sentence. they both means the action is finished or completed. It is past tense. So, 我吃了巧克力 and 我吃巧克力了 just means I ate chocolate.
But there is a slight difference between them . 我吃了巧克力 means to emphasize the verb ate.
我吃巧克力了 means to emphasize ate chocolate. Hope it helpful.
This will not answer your question but shed some light to the mystery of the 了 particle.
了 does not mean past tense. It denotes **CHANGE**.
This sentence points out no change. It only says that once you were living there.
Your friend calls from the kitchen: 吃饭了！
This one points out a change, namely that we are ready to eat the meal. It is not ...
It seems to me 沒有 is more formal, and 沒 is ellipsis form. I don't think they exhibit difference in usage, though. Note that 沒有 may either be "do not have/possess" (something) or "not" (adverb), depending on context. Chinese is pretty vague sometimes. Lastly, remark that, following the instance 有殺人, “有” is not mandatory and serves as a stressing word.
以为 is mostly used for:
like: 我以为你死了！ I thought you were dead!
觉得 is closer to:
feel/think (of an opinion)
like: 你这样说，我觉得非常不靠谱 What you said is totally unreasonable.
想 is close to the English:
like: 我想这个案子肯定有猫腻。 There's something fishy about this case.
In Chinese the idea of English 'think' is hardly ever used - even ...
I'd say the two constructions are used in different contexts. See below example:
I'm not quite behind the idea that 了 is 'factuality marker', while 是…的 is. 了 is an indicator of completion.
I feel that 是……的 is not an independent structure. I prefer to parse it as 是 （……的）. 的 is actually a marker of the descriptive chunk before it, transforming it to （I am not sure of that jargon, but something adding 的 to form a noun phrase or else thing is very common）. 是 express quality, feature, etc of the subject in a general sense. When I say general sense, ...