Learn Spanish (Latin America): No puedo ir a la fiesta.

When two verbs work together to express one idea, only the first verb is conjugated (changed to match the person). Since puedo already tells us "I can", the second verb stays in its dictionary form (the infinitive), which is ir (to go). This is the standard rule for "poder" + infinitive.

Where is the word for "I" (yo) in this sentence?

In Spanish, you can usually leave out subject pronouns like yo because the verb form tells you exactly who is doing the action. The word puedo specifically means "I can", so saying yo no puedo is perfectly fine but unnecessary unless you want to emphasize that you are the one who cannot go.

Why does the verb poder change its spelling to puedo here?

Poder (to be able to) is what we call a "stem-changing" verb. In the present tense, for the "I", "you", "he/she", and "they" forms, the o in the middle of the verb changes to ue. That is why we say puedo instead of "podo".

What does the a do in ir a la fiesta?