Hi RyanEx: The first thing I did was work for a computer company for three years.In English, as is the case with most human languages, only one verb per simple sentence is allowed to carry tense. In our example sentence, there are two simplex sentences. The verb 'did' carries tense in the first sentence and the verb 'was' carries tense in the second sentence, which means the verb 'work' can't.
Here's how the syntax works:
Sentence 1: I did the first thing.
Cleft: The first thing (that) I did...
Sentence 2: I worked for a computer company.
Combined: The first thing that I did / was work for an computer compant.
Note that, this is a cleft structure, so a form of BE (i.e., was) is inserted between the sentences.
P.S. I adore your site. (I also teach in Shanghai).