In addition to the general rules Bozyo mentioned above when conjugating Spanish verbs in the future tense, there are some irregular verbs that use a modified form of the infinitive. The following verbs drop the letter e from the infinitive ending:

caber (to fit into)
poder (to be able)
querer (to want)
saber (to know)

Example:

caber
cabré
cabrás
cabrá
cabremos
cabréis
cabrán


In these verbs, the vowel of the infinitive endings -er and -ir changes to d:

poner (to put)
salir (to go out)
tener (to have)
venir (to come)

Example:

poner
pondré
pondrás
pondrá
pondremos
pondréis
pondrán


In these verbs, the letters e and c are dropped from the infinitives before adding the future-tense endings:

decir (to say)
hacer (to do)

Example:

hacer
haré
harás
hará
haremos
haréis
harán