Formation of verbs: present simple tense

INDEX
1. Introduction 2. Present simple tense formation: -AR verbs 3. Present simple tense formation: -ER verbs 4. Present simple tense formation: -IR verbs

1. Introduction
In Spanish all verbs belong to one of the following three groups: -AR, -ER or -IR (e.g. cantar, comer, vivir). The different verb tenses are normally formed by adding different set of endings to the stem of the verb.
2. Present simple tense formation: -AR verbs

The present simple tense of verbs belonging to the -AR group is formed in the following way:

Cantar (to sing)

Stem | Ending | Present
cant- | -o | cant-o
cant- | -as | cant-as
cant- | -a | cant-as
cant- | -amos | cant-amos
cant- | -áis | cant-áis
cant- | -an | cant-an

Regular present simple tenses of verbs belonging to this group such as estudiar (to study), nadar (to swim), etc. are formed as above.
3. Present simple tense formation: -ER verbs
Endings for the regular present simple tense of verbs belonging to the -ER group are as follows:
Comer (to eat)

Stem | Endings | Present
com- | -o | com-o
com- | -es | com-es
com- | -e | com-e
com- | -emos | com-emos
com- | -éis | com-éis
com- | -en | com-en

Regular present simples tenses of verbs belonging to this group such as beber (to drink), vender (to sell), etc. are formed as above.
4. Present Simple Tense Formation: -IR verbs
The present simple tense of verbs belonging to the -IR is formed in the following way:
Vivir (to live)

Stem | Endings | Present
viv- | -o | viv-o
viv- | -es | viv-es
viv- | -e | viv-e
viv- | -imos | viv-imos
viv- | -ís | viv-ís
viv- | -en | viv-en

Regular present simple tenses of verbs belonging to this group such as partir (to set off), agredir (to attack), etc. are formed as above.