Verbs Left Unconjugated
 

I've been working on verb conjugation. I started with the present tense, then the preterite and am now on the imperfect. I'm starting to notice the conjugations in those tenses when I read Spanish.

But I've also noticed mixed in some sentences with conjugated verbs are others that are left unconjugated. Is there a reason some verbs are conjugated and others are not?

For example:
Vamos a pasar el mes de enero en el sur.

"Pasar", spend, is left unconjugated. I noticed several other sentences like this.

I think it's because "pasar" is being used in the infinitive form similar to how we would say in English. "we are going to spend. . "

An excellent question, and Caliber1 has it: in your example "pasar" is in the infinitive, it functions as a noun, and it is the complement of the construction "ir a [infinitive], which is an extremely common alternative to using the future tense.

Many verbs either allow or require some type of complement in order to complete the idea of the verb. Often one of the allowed or required types of complements is an infinitive. The types of complements that can follow a particular verb and how each type of complement connects to the verb (either directly or after a particular preposition such as 'a', 'de', 'en', 'por', 'para', 'con', 'contra' and so on) are specific to each verb, which means that you have to learn them one by one.

Can I just add that a clause, by definition, must contain exactly one finite verb. A finite verb is conjugated. The infinitive is a non-finite verb, which doesn't conjugate. Your conjugated verb is vamos.