Learn German: Der Techniker ist nicht höflich, aber er löst das Problem. - The technician is not polite, but he is solving the problem.

In German, adjectives only get endings when they come directly before a noun (like ein höflicher Techniker). Because höflich (polite) comes after the verb ist to describe the subject, it stays in its base form with no ending.

Does the word aber change the sentence structure in the second half?

No. aber (but) is a coordinating conjunction. You can think of it as taking up "position zero" in the sentence. This means the clause that follows it keeps normal word order: the subject (er) is in position 1, and the conjugated verb (löst) stays in position 2.

Why is nicht placed exactly where it is?

When you want to negate a specific adjective, nicht goes right in front of the word it is negating. Since we are specifically saying he is not polite, nicht sits directly before höflich.

What is the dictionary form of the verb löst?

The infinitive is lösen (to solve). It is a regular verb, so to conjugate it for er (he), we just drop the -en and add a -t, making it er löst.