I remember reading that some verbs don't accept 了 or 过.
Side question： When I use a word like 昨天 or 以前 do I use 了 or is it redundant? e.g. 昨天我去买了一台电脑 or 昨天我买一台电脑
过 and 了 are aspect markers, and can be used with any verbs, including 过 and 了 themselves.
过 is the experential aspect, indicating that an event has taken place (or the negation thereof), regardless of timeframe (there is no tense in Chinese). 我去过巴黎二十次。 (I have had the experience of visiting Paris twenty times.) 我没去过越南。 (I have not had the experience of going to Vietnam.) 我从来没看过电影。 (I have never had the experience of seeing a movie.)
了 is the perfectative aspect, indicating that an event has been completed, again regardless of time. 我买了个新脚踏车。 (I have bought a new bike.) 吃了饭以后可以逛逛街。 (We can go for a walk after we have finished eating [future]).
Both aspects can be expressed simultaneously: 我吃过了。 They can also be used with other complemented verbs: 我看得见过了。
Aspects can be used with any verb. The only restriction is context and combining conflicting aspects. For example, using durative 着 with completion aspects 过 and 了 makes no sense.
As for your side question: since there is no tense in Chinese, it does not matter in which time frame these aspects are used. The use of 昨天 or 以前 indicates the time, whereas 过 and 了 indicate completion, regardless of time.
(A bit late for this, but... how I wish to gain reputation! ^ ^ )
I think the distinction is a soft one, rather than compulsory.
When you stress you have done something and there is some action or change in the process, use
When you stress you have completed doing something, use
了; it is somewhat like perfection, but see below.
For brevity, call
(a)昨天我去买一台电脑 (b)昨天我去买了一台电脑 (c)昨天我去买過一台电脑
(a) is okay but rare, sounds a bit dry. It reminds me of a very formal narrative without sentiment (in a fiction), or a formal speech.
(b), however, is very common. It occurs to me German usage of auxiliary
haben "have" (or
sein for a group of verbs such as
kommen) is also soft: it does not strongly stress perfection, but loosely equivalent to English past tense.
(c) is okay but somewhat dubious, probably because
買 occurs within a short moment, and little is "taking place".
But consider the context: You and your friends are talking about a computer sales in that store who is very bad-tempered, and you say,
昨天我去買過一台電腦，店員真的好兇啊! you are stressing you really have seen him, and he is really bad-tempered. This is perfect. You have tried something, and found something new.
I can think of a good example: as of
愛 (following Enrico's example), one usually says
我愛他, but it is odd to say
我愛過他. Why? because it changes little visible to "love" someone.
But does it have its own appropriate context? yes! Consider: A and B are talking about C. Now A reveals to B that A and C once had a relationship, but broke up.
Still somewhat remorseful, A remarks:
我愛過他 It is appropriate then. (as a side remark,
愛 is so formal-sounding and really uncommon in spoken Mandarin, no matter parental love or romantic love....at least in Taiwan I think.)
Many of my students have this question. And what you are trying to ask is not which verbs don't use 了, but which verbs don't use 了 to express past tense.
Basicaly Mandarin "separates" verbs into action verbs and stative verbs. So, any verb that is not an action, for example, 有 是 在 etc don't express past tense using 了.
Here's a small list. Since I'm not a native speaker, I'm not sure what is an action or not in Chinese. Hopefully someone can help us make a more detailed one:
"mental" verbs: 知道、喜欢、爱、etc.
modal verbs: 应该、要、会、能、可以、etc.
If the above verbs take 了, it indicates a change of state. For example:
我是大学生了。 I'm a graduate student now.