Specific て vs た usage

Japanese
#1

Consider the following sentences.

(1) 先生が説明しおわったから、先生に質問をした.
(2) 彼はパソコンを買いたがって、週末も働いた。

In (1), Bunpro does not allow for “おわって”, and I don’t really know why. I thought て could be used here because the tense was dictated by the final “した” anyway.

In (2), the opposite happens: Bunpro allows “買いたがって”, but not “買いたがった”. The intended meaning is “He wantED to buy a PC, so workED weekends.” It’s all in the past, so I don’t know why て-form must be used.

Both て and た seem to work in these sentences for me, so there must be something else I don’t understand.

#2

I don’t know for sure about the second one, but in the first one if you replace た with て, you get a correct sentence with a different meaning. This means which one of the two is counted correct is based on how the sentence is translated. て before から indicates a temporal relation (after), while た indicates a causal relation (because).

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#3

From my undestanding ‘てから’ implies a grade of control by the speaker in the secuence of the events. One could said that the person in (1) doesn’t decides when the explanation is going to end or has part in deciding.
Like Donutmancer points out is more a scenario of causality.

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#4

As @Donutmancer says, “て before から indicates a temporal relation (after), while た indicates a causal relation (because)”.
In this case, I understand that the expected answer considers that there is a causal relation: you can ask the teacher a question because he has finished with the explanation. Therefore, おわったから has to be used.
(Would you use おわってから you would just imply that the teacher finished with the explanation and then you put him a question.)

In this case you wouldn’t be allowed to say たがった as you are ‘in the middle’ of the sentence and need the conjunctive て-form to join the first part of the sentence to the second one. In this sentence, the て-form will serve to express the reason or cause, as explained here.

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#5

That makes sense. Thanks everybody for the help!

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