To conjugate a verb means that you add some extra information to the infinitive. If you do not conjugate it, you need a personal pronoun in order to add this information. Let's explain it with an example.
They swim.
In this case it is very clear who is swimming, they. If you don't add the pronoun, you only know what someone is doing, but you don't know who is doing it, swim alone does not tell you who is swimming. The situation changes if you have something like
swims.
In this case you know that he, she or it is swimming. You may think that this is irrelevant, but here the author doesn't agree because there are languages, which indicates who is actually doing the action described by the verb through conjugation, for example Italian. In Italian every person has its own declension, meaning there is a different ending, depending on whether we are talking about 'I', 'you', 'he', 'she', 'we', 'you', 'they'.
So there is no need to put a pronoun and normally there is no pronoun in front of the verb. We are still going to use pronouns for a while, but we will stop doing it later.
verbs ending on -are
infinitive
translation
parlare
to speak
comprare
to sell
saltare
to spring
verbs ending on -ere
infinitiv
translation
vendere
to sell
vedere
to see
credere
to believe
verbs ending on -ire
infinitiv
translation
partire
to leave
aprire
to open
soffrire
to suffer
To conjugate a verb it is necessary to know the stem, because the respective endings are added to the stem. You get the stem if you take away the endings of the infinitive (-are, -ere, -ire).
verbs on -are
verb
stem
ending
parlare
parl
are
comprare
compr
are
saltare
salt
are
verbs on -ere
verb
stem
ending
vendere
vend
ere
vedere
ved
ere
credere
cred
ere
To the stem is added the ending and these endings are different for each type of verb.
verbs on -are
verbs on -ere
verbs on -ire
stem
ending
stem
ending
stem
ending
parl
o
vend
o
part
o
parl
i
vend
i
part
i
parl
a
vend
e
part
e
parl
a
vend
e
part
e
parl
iamo
vend
iamo
part
iamo
parl
ate
vend
ete
part
ite
parl
ano
vend
ono
part
ono
And know we can built our first simple sentences.
Some sentences
Io compro la casa. => I buy the house.
Noi vediamo la ragazza. => We see the girl.
La ragazza mangia una mela. => The girl eats an apple.
I ragazzi scrivono una lettera. => The boys write a letter.
Le donne aprono la porta. => The women open the door.
Voi vendete la vostra macchina. => You sell your car.