Learn Spanish (Latin America): Acababa de contestar, pero tuve que colgar por el wifi pésimo. - I had just answered, but I had to hang up because of the awful Wi-Fi.

The phrase acabar de plus an infinitive means to have just done something. When you use it in the imperfect tense, like acababa de contestar, it translates to "I had just answered." It sets the background scene in the past. The preterite (acabé de) is rarely used this way; it would sound more like "I finished answering."

Why switch to the preterite tuve que right after using the imperfect acababa de?

This sentence perfectly shows how the imperfect and preterite work together. Acababa de contestar (I had just answered) sets the background context. Tuve que colgar (I had to hang up) is the sudden, completed event that happened within that background.

Why use por instead of porque to say "because of"?

To say "because of" followed by a noun (like el wifi pésimo), you must use por. If you wanted to use porque (because), you would need to follow it with a conjugated verb, like porque el wifi era pésimo (because the Wi-Fi was awful).

Is pésimo just another word for malo?