"No puedo trabajar mañana."
Translation:I cannot work tomorrow.
Trabajo is a conjugated verb in the yo form, and so is puedo. You can almost never have two conjugated verbs together. Here it would be something like saying [I cannot I work].
So the second verb needs to be an infinitive (the unconjugated form ending in -r).
No puedo trabajar. I cannot work.
Tengo que estudiar. I have to study.
Quiero aprender mucho. I want to learn a lot.
Are there any particular rules around when we can drop the "to" from the infinitive? "to work" vs "work". Is it a special thing when following poder, or do we do the same when following any verb where dropping the "to" would sound more natural?