Learn Swedish: Det gäller att avleda vaktchefen om han skulle se oss. - It is crucial to distract the head of security if he were to see us.

Using om + skulle + bare infinitive (se) creates a tentative conditional. It translates roughly to "if he were to see us" or "if he should see us". It makes the hypothetical situation feel more distant, hypothetical, or less likely than using a simple present tense like om han ser oss (if he sees us).

I learned that gäller means "to be valid." Why is it translated as "It is crucial" here?

While gäller does mean "to be valid" (like a bus ticket or a passport), the specific phrase det gäller att is a very common fixed expression. It means "it is important to", "it is necessary to", or "it is crucial to".

What is the literal makeup of the word avleda?

It combines the particle av (off or away) with the verb leda (to lead), so it literally means "to lead away." In Swedish, this is the standard vocabulary for "to distract" or "to divert" someone's attention.

How would the word order change if we put the om clause at the very beginning of the sentence?

Moving the om clause to the front would trigger the V2 (verb-second) rule in the main clause. Because the entire om clause acts as the first block of information, the main verb gäller must come immediately after the comma: Om han skulle se oss, gäller det att avleda vaktchefen.