Usage of modal verbs

Hi everybody!
My name is Marcela and I´m from Argentina. I have some doubts as regards modal verbs. For instance, I need to know how to identify the different modalities of “CAN”; for me, it is specially difficult to identify whether it is acting as a possibility or an ability / capacity on certain occasions. How can I do to get the real meaning of this modal verb as a possibility or an ability / capacity when the situation is not so “clear - cut”?
Thank you very much,
Marcela

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To get the meaning you must understand the context. There is no other way.
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Hello Mister Micawber!
Thank you for your reply. You are right, but sometimes it doesn´t prove that easy for me. For example, I saw (I don´t remember exactly where) that the following sentence: “He can´t afford a new car” is an example of “ability”; why not impossibility? or even capacity? For me it isn´t ability at all. And, for instance, take these two sentences, one in active voice “Even children can play this game” and the other in passive voice: “This game can be played by children”. I found that the former is taken as an example of “ability” but the latter as an example of possibility because of the passive meaning. Nevertheless, there are teachers who think that both have the same meaning, “ability”. What do you think?
The best,
Marcela

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As I said, it is a matter of context.

This game can be played by children-- it is easy.
This game can be played by children-- it is not X-rated.