BBC Learning English would do Lyrics

Hi Mohammed, thanks for your question.

The main point to consider here is that 'does' and 'doesn't' are all present simple forms of the verb 'to do'. One of the main uses of the present simple is to express a general truth, a fact, habit or routine when we're not only thinking about now but thinking in general. In your example, you say He doesn't know - this suggests to me a simple fact or general truth about his situation, in the same way that if I say I don't own a cat or He doesn't like ice-cream this is simply a general fact or truth without any particular time attached to it.

'Would' can be used in several different ways, but to relate my answer to your specific example, He wouldn't know, we're using it in this case to hypothesise or imagine how much - or conversely - how little he knows. Here we don't think it's very likely that he would know, but it's not as definite as using the present simple form 'doesn't know' where you feel absolutely sure about the limits of his knowledge. So one of the most common uses of 'would' or 'wouldn't' is to hypothesise or imagine a situation or action. If we want to express this in the past, we have to use 'would' or 'wouldn't' with 'have' and the past participle - in this case 'wouldn't have known' or 'wouldn't have done.' For example:

There was no point asking him yesterday because he wouldn't have known then.

These are certainly not the only uses of 'would', but I hope, at least, this answers your question about the functions of 'would' as it appears in your sentence.