Learn Swedish: Titta på kretskortet och säg vilken kabel som är rätt. - Look at the circuit board and tell me which cable is right.

This is a classic Swedish rule for indirect questions. When a question word or phrase (here, vilken kabel) is the subject of the subordinate clause, you must add som after it. If the question word were the object, like in "tell me which cable you chose" (säg vilken kabel du valde), you would not use som.

Why is the verb written as säg instead of säga?

The sentence gives a direct command, so it uses the imperative form of the verbs. While titta looks identical to its infinitive, the imperative of säga is the short root säg.

Is kretskortet an en or ett word, and how is it built?

It is an ett word, which is why it has the definite ending -et. In Swedish compound words, the final noun always determines the gender. Here, en krets (a circuit) combines with ett kort (a card or board) to make ett kretskort.