are you suppose to ignore caluses set off in commas?

You cannot ignore in all the cases. You need to take a subjective call. Understanding meaning of the sentence will be more important. You will start getting better as you practice.

This is a very interesting topic to bring up! There is a GMAT grammar rule that relates to this, but it's more complex than what you've described here. I'll just say what it is first, and then explain it:

When you're trying to find the main subject and verb of a sentence or clause, you can ignore modifiers.

Okay, why would you want to find the main subject and verb? There are two reasons. One, you might be trying to see whether the subject and verb agree in number. Here's a pair of example sentences:

University employees, who are frustrated with the limited local options available after the closing of the cafeteria on campus, is requesting a longer lunch break.
University employees, who are frustrated with the limited local options available after the closing of the cafeteria on campus, are requesting a longer lunch break.

You might see a split like this one on the GMAT: one sentence includes the singular verb 'is,' while the other includes the plural verb 'are.' Since 'is' or 'are' is the main verb in this sentence, you'll need to find the main subject and determine whether it's singular or plural. To do so, you can ignore the lengthy modifier between the commas. However, you don't ignore it just because it's between the commas! You ignore it because it's a modifier. Sometimes, modifiers occur between two commas. At other times, modifiers occur at the beginning of a sentence, or at the end of a sentence, or without any commas at all.

In your sentence, there are two modifiers. One is at the beginning of the sentence, and the other is at the end. If you wanted to find the main subject and verb of the sentence, you could ignore those modifiers.

To get to his house, Jim biked along an old dirt road,which cut through the woods.

By the way, how do you know that those two parts of the sentence are modifiers? The first one starts with the preposition 'to,' and the second one starts with the relative pronoun 'which.' The bit in the middle, which has no 'modifier markers,' is actually the main body of the sentence. The main subject is 'Jim' and the main verb is 'biked.'

Two, you might cross off modifiers if you were wondering whether the core structure of the sentence was correct. For example, here's a sentence with a bad core:

The boys, who enjoyed playing with action figures, which they purchased at the dollar store.

If we start crossing off modifiers, we end up with a very short 'sentence':

The boys, who enjoyed playing with action figures,which they purchased at the dollar store.

'The boys' isn't a complete sentence by itself, so we know that the overall sentence must be incorrect.