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In the cases below, do I always have to use 'may' instead of 'can'? Both sound correct to me however, I had read explanations that said that 'may' should be used after 'so that'.

I came earlier so that I may help them. I came earlier so that I can help them.

Please be on time so that we may leave before 5. Please be on time so that we can leave before 5.

Thanks

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    In the past tense you'll need to use could not can, might not may. – tchrist♦ Jan 13 '18 at 11:24
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The OP says, "I had read explanations that said that 'may' should be used after 'so that'". I don't think there is such a hard and fast grammar rule.

Can and may can be used to express possibility and permission with some differences.

Possibility: (from English Grammar Today, Cambridge Dictionary)

When we talk about possibility, we use can, could and may, but they are different in meaning.

Compare:

It can be dangerous to cycle in the city. (This expresses what the speaker believes is a general truth or known fact, or a strong possibility.)

It could/may be dangerous to cycle in the city. (This does not express a general truth. The speaker is only expressing a weak possibility.)

Permission:

We use can, could and may to ask for permission. We use can and may, but not could, to give permission. May is less common:

Compare:

Can I ask you a question? ( informal )

Could I use your phone? (more formal/polite)

May I use your phone? (even more formal/polite)

Also from Oxford Living Dictionaries:

‘Can’ or ‘may’?

People are often uncertain about whether there is any difference between can and may when these verbs are used to ask for or grant permission. For example, is one of these two sentences ‘more correct’ than the other?

Can I ask you a few questions?

May I ask you a few questions?

There is a widespread view that using can to ask for permission is wrong and that it should only be used in expressions to do with ability or capability, e.g.:

Can she swim?

Can you speak Italian?

But the 'permission' use of can is not in fact incorrect in standard English. The only difference between the two verbs is that one is more polite than the other. In informal contexts it’s perfectly acceptable to use can; in formal situations it would be better to use may.

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Can means you're able to do something.

May means you're permitted to do something.

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  • I'd like to know in this exact context if both are permitted. – Rebecca Viaro Piassa May 18 '17 at 0:08
  • In informal usage, either will work. May tends to sound a little more formal, but it's not, really. In your second example, you could get away with either, but can is the better choice -- Please be on time so that we are able to leave before 5. versus Please be on time so that we will be allowed to leave before 5. – Roger Sinasohn May 18 '17 at 0:13
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    Your first example, however, is not a good one -- the tenses don't match. I should be I came earlier so that I might help them. or I came earlier so that I could help them. Alternatively, you could say I will come earlier so that I may/can help them. Unless you're going to be punished by not being allowed to help if you don't show up early, you probably still want can. – Roger Sinasohn May 18 '17 at 0:15

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