Notes from ¿Qué tal? - Infinitives and Personal Endings

The infinitive (el infinitivo) of a verb indicates the action or state of being, with no reference to who or what performs the action or when it is done (present, past, future) – In English the infinitive is indicated by t-ar, -er, or -ir.
To conjugate (conjugar) a verb means to give the various forms of the verb with their corresponding subjects: I speak, you speak, he (she, it) speaks, and so on. All regular Spanish verbs are conjugated by adding personal endings (las terminaciones personales) that reflect the subject doing the action. These are added to the stem (la raíz or el radical), which is the infinitive minus the infinitive ending: habar ⇒ habl-x
The following personal endings are added to the stem of all regular -ar verbs: -o, -as, -a, -amos, -áis, -an.
In Spanish the meaning of the English word for is included in the verbs pagar (to pay for) and buscar (to look for); to is included in escuchar (to listen to).
As in English, when two Spanish verbs are used in sequence and there is no change of subject, the second verb is usually in the infinitive form.