so that S + can, may, will ....

I learned that 'so that S + can (may, will ) means in order for S to do.....
Then I've found the sentences without can, may will.

I'm doing my homework on the bus so that I don't get punished.
I like to arrive 20 minutes early so that I have time for a cup of tea.

'can may, will' are not necessarily needed?
or is there a difference of meaning? just literally different?

1. I like to arrive 20 minutes early so that I have time for a cup of tea.
2. I like to arrive 20 minutes early so that I can have time for a cup of tea.
3. I like to arrive 20 minutes early so that I will have time for a cup of tea.
4. I like to arrive 20 minutes early so that I may have time for a cup of tea.

brightsun17 said:

I learned that 'so that S + can (may, will ) means in order for S to do.....
Then I've found the sentences without can, may will.

I'm doing my homework on the bus so that I don't get punished.
I like to arrive 20 minutes early so that I have time for a cup of tea.

'can may, will' are not necessarily needed?
or is there a difference of meaning? just literally different?

1. I like to arrive 20 minutes early so that I do have time for a cup of tea. I added 'do' to make all the sentences analogous. It wouldn't normally occur there.
2. I like to arrive 20 minutes early so that I can have time for a cup of tea.
3. I like to arrive 20 minutes early so that I will have time for a cup of tea.
4. I like to arrive 20 minutes early so that I may have time for a cup of tea.

Click to expand...

Obviously they all have the same basic meaning. Arriving 20 minutes early gives you time for a cup of tea.
But the connotations are all different.
3. is wrong grammatically. Your sentence is about a habitual action. "Will" doesn't really work.
4. is not very natural.
2. is probably the most natural way of saying it. You arrive early, so you are able (can) have a cup of tea.

You really need to approach all of these verbs separately and learn their meanings. It's a significantly complicated task, and not one we can do for you in a forum like this. Besides, there are good grammar books that can explain those auxiliary and modal verbs.
If you have a specific question, we'd be happy to answer it.

I find 2 quite unnatural: "I like to arrive 20 minutes early so that I can have time for a cup of tea". I find the combination of "can" and "have time" doesn't work.
I would use either:

I like to arrive 20 minutes early so that I have time for a cup of tea.
or
I like to arrive 20 minutes early so that I can have a cup of tea.