Learn Spanish (Latin America): Yo estoy cansado y no puedo correr hoy.

When using poder (to be able to / can) to say you can or cannot do something, only poder changes to match the person (puedo). The second verb always stays in its unconjugated dictionary form (the infinitive). So, I can run is puedo correr.

I know the verb for to be able to is poder. Why does it change to puedo here?

Poder is a stem-changing verb. This means the vowel in the middle of the word changes when you conjugate it. For poder, the -o- changes to -ue- for most forms in the present tense (I, you, he/she, they). So for yo (I), it becomes puedo.

Where exactly does the no go when you have a phrase like puedo correr?

The word no goes right before the first conjugated verb. In this sentence, it goes right before puedo. You never put the no between the conjugated verb and the infinitive, so it is always no puedo correr.

Why does this sentence use estoy instead of soy for I am?