/kæn/ is the stressed form. This is used: 1) When 'can' is not followed by another verb. For example: You can't speak German, can you? /kæn/ Yes I can. /kæn/ 2) When you particularly want to emphasise the 'can'. For example: You can't speak German, can you? /kæn/ Yes I can speak German! /kæn/ In fact, I grew up in Germany. /kən/ is the unstressed form. You use this when it is followed by a main verb and the main verb is more important than the 'can', so it isn't emphasised. What can you do to help in the house? /kən/ Well, I can cook and I can sew. /kən/ Here's a joke. It's a word play on the word 'handy'. 'Handy' means convenient or within easy reach, and a 'handyman' is a person who helps with general maintenance and manual work. All of the 'cans' are unstressed /kən/, apart from the last one, which is stressed /kæn/. I've come to apply for the job of handyman. OK, what can you do? Can you do carpentry? No. Can you do you plumbing? No. Can you do electricals? No. Can you do painting and decorating? No. So what CAN you do? Nothing. So what 'handy' about you, then? I live next door.

When the stress falls on "can", you read it as /kæn/. When it's in a weak position, you use /kən/. Can I help? Yes, you can. (Strong positions: /kæn/ ) I can help you. (Weak position: /kən/ )

Your question is not phrased correctly. You need to use the aux verb 'do' in the simple tenses for questions. When DO we read "can" as /kæn/ and when DO we say "can" as /kən/? Without the auxiliary verb it's just an incomplete clause like "When we get to the market and when we get out of the car...(you need to complete this sort of phrase). The form is like this: When one and/or another thing happens, (then something else will happen). You will probably get a lot of different opinions about this, especially when the word is pronounced out loud. I'm American so I'm answering from that perspective and experience. I think it is really a matter of personal preference as to when to use kæn vs kən. The only answer I really agree with is Suki's number two - when you really want to emphasize your ability to do something. But keep in mind that volume and intonation are also used with this sort of emphasis, it's not just about the vowel sound. You can add emphasis with either pronunciation. So really, it doesn't much matter what sound you use, in your head or when speaking out loud.