Subordinate clause not equivalent to main clause

Question

The head teacher of the institute ________ an M.A in English so he can also teach us.
1) did
2) has done
3) have done
4) has doing

I think the problem I'm facing is 'so'. Here, 'so' is a subordinating conjunction. The rule of subordinating conjunctions says it should be grammatically equal between the main clause and the dependent clause. If I put any of the options as an answer, it will be wrong, because, "so he can also teach us" is in present form, and the options given are in either simple past or present perfect.

Answer 1

Options 1 and 2 are both correct. Both state that the teacher received a degree in the past and so is eligible to teach currently. Option 3 is incorrect because it is plural and "teacher" is singular. Option 4 is not grammatical -- it would have to be "has been doing."

It's worth noting that, in written English, to receive a degree would be preferable to do a degree, which is informal.

Answer 2

@Ringo gives a good explanation of the correct answer to the test question, so I won't repeat that.

I think the question is flawed because, (a) there appear to be two right answers. Choices 1 and 2 are both valid. And (b) "did an MA" is very informal. As Ringo says, it should be "received an MA" or in this case I would prefer "earned an MA".

As to your second paragraph, it is very common for a sentence to mix tenses when things happen at different times. In this case, the teacher did something in the past -- "did" a degree -- so that he is able to do something else in the present -- teach us. You can't make them the same tense because to make sense in context, the earning of the degree had to come before he could teach. He couldn't earn the degree and teach at the same time. Or at least, that would be unlikely.

Answer 3

If that is the rule which they are testing, then "has done" is more correct than "did". "Has done" is present perfect, which agrees with present tense in the subordinate. It's also semantically a better fit since the use of the present perfect is used when an action has been completed in the past, but its effects are still in the present. In a sense, you can see it as a descriptor of the present effect, rather than the completed action. The implication of this tense is that M.A. is still valid, which is supported by subordinate clause: they can teach because they hold an MA.