Recently i saw such sentences as "火车站的东西真不能买” and " 以后的事，以后再说；而不能肯定的事不要说” 。 I know that 能 means " be able to... " but I'm confused in these sentences . Can you explain its meaning to me in more detail?
First, translations for your examples.
火车站的东西真不能买。= (You) should not buy things from train stations.
不能肯定的事不要说。= Don't say the things that you are not sure about.
According to the dictionary, your understanding is mostly correct. "能" by itself means "able", "be able to", "ability", etc. However, also note that these sentences are oral language, so they are more or less derives from daily experience and simplifications.
"不能买". This is a common phrase that is used to express forbidding. Equivalent to "不要买" but with stronger emotion against the action. Another example: "人不能不讲道理。"= One should not be unreasonable.
"不能肯定" simply means "cannot affirm", i.e., "not sure", where "cannot" approx. equal to "not able to".
as always context is most important in chinese, and the meaning of these phrases may vary a bit depending on the situation. lets go ahead and address them in some examples to help understand a base comparison:
不能 most common use is to mean "cannot", its also possible to mean "should not" in some context. here it is fairly straightforward "the stuff at the train station really cannot/should not be bought"
your second sentence is using a cannot definition as well, "cannot be sure"
you do not have an example sentence for context on 不用, so I will simply say that it can very commonly mean "not needed", such as the example 不用搭車-- no need to drive/ride a vehicle etc.
I save 買不到 for last as the most complicated from a grammar point of view. just in case here is a quick review of result complements:
certain verbs do not tell you anything about the result of the action. a special secondary verb can be added as a complement to show this result. 到 is a common way to show completion of an action when in a complement.
what we have here is a negative result complement. so this phrase can be read as "did not buy".
Other answers have already explained quite well, let me give some more insights.
能 means be able to, can, which emphasize the ability the one who perform the action. 要/应该 means will/should, which emphasize the willingness of the one who perform the action or the willingness of someone else (sometimes the one who talks to you, sometimes implicitly refers to a common sense) that imposes to the one who actually perform (or try to perform) the action.
Normally, when you try to persuade someone (or oneself) not to do something, you express the willingness. However when you insist strongly, as if the one doesn't have the ability to do that. This means pretendedly, you deny the action not by your willingness but by one's lack of ability.
买不到 emphasize the objective fact that there's no way to buy it. (you cannot find it in the market) 到 has the meaning of reach, there's no way to reach it by buying. Similarly to 来.
不用买 not necessary, no need to buy it, there's nowhere we can use it.
Maybe one more expression for fun, you can use 买不起 to mean you can not afford it. 起 has the meaning of start to do something.
Question on the word 不能. Can you explain its meaning to me in more detail? For example, "火车站的东西真不能买” and "不能肯定的事不要说”。
The Chinese word 能 has four basic meanings.
(1) 能 = able to, have the capability (resources, energy, force, power, skill, knowledge, etc.) to do something.
(2) 能 = can (could, be willing to) do something.
(3) 能 = to be allowed to (have permission to) do something.
(4) 能 = to happen (or exit, be realized).
The meaning of its opposite 不能 could be an opposite of the above usages of the word 能. 不能 usually has three levels of "disagreement".
(1) 不能 = can not (not able to, not willing to)
(2) 不能 = should not (It's better not), as advice or suggestion.
(3) 不能 = must not (you'd better not, you are not allowed to), as strong advice or warning.
(1) 火车站的东西真不能买 = You'd better not buy anything at the railway station. = The goods sold at the railway station are either expensive or counterfeit stuff.
(2) 不能肯定的事不要说。= It's better not to say anything that you are not sure for yourself.
You are correct that 能 means "be able to", or "can", which is the case in the second sentence,
- "...不能肯定的事不要说” - ...don't say the thing that is not able to ascertain.
For the tricker first sentence, we first need to recognize it is a warning/advice to "not to do something" rather than talking about the "ability/able to do something", then ask what "不能买" mean.
We recall that an equivalent term of 能 is "可以(may)", so the sentence can be expressed as "...不可以买", and translated to "...may not buy". At this point, we further recognize that "may not do" can literally mean "should not do", so the correct translation is:
- "火车站的东西真不能/可以买” - In fact (真), (one) should not purchase anything/things sold in the train station.
In the context of the sentence "火车站的东西真不能买" (huǒchē zhàn de dōngxi zhēn bùnéng mǎi), "不能" (bù néng) is not used to describe ability or competence, instead it means "should not""not advisable" ," indicating that it is not a good idea to buy things at the train station.
这里的饮料不能喝。(Zhèlǐ de yǐnliào bùnéng hē.) You should not drink the drinks here.
他们的建议不能听。(Tāmen de jiànyì bùnéng tīng.) You should not follow their advice.
这个地方不能去。Zhège dìfāng bùnéng qù. You should not go to this place.
Hope this can help.