You have to know for each verb the bases of the infinitive, of the "Indicatif présent" and the past participle.
After you will just have to study a few irregular bases for the "indicatif futur simple" and the "subjonctif présent" and it will be all. You will master the French conjugaison.
There are four main types of French verbs.
Regular verbs with only one base.
The majority of the French verbs and nearly all the -er verbs belong to this category.
To find the regular base, take the infinitive, take off the symbol of the infinitive (-er/-ir/-re/-oir) and here is the base.
By example, for the verb "to speak" :
parlerthe regular base is
parl- for all the persons
Verbs with two bases.
A base for the three singular persons (
je, tu, il/elle/on)
another for the three plural persons (
nous, vous, ils/elles)
By example, for the verb "to speak" :
partirthe
singular base is
par-
the
plural base is
part-
But also the verbs :
dormir (dor-/
dorm-)
servir (ser-/
serv-)
sortir (sor-/
sort-)
battre (bat-/
batt-)
mettre (met-/
mett-)
connaître (connaî-/connaiss)
lire (
li-/lis-)
écrire (
écri-/écriv-)
conduire (condui-/conduis-)
plaire (
plai-/plais-)
suivre (sui-/
suiv-)
vivre (vi-/
viv-)
craindre (crain-/craign-)
peindre (pein-/peign-), etc.
As you may notice, the plural bases are very often regular!
A lot of the verbs of this category are verbs in -ir of the
"finir" type
(the dictionary will tell you the type of -ir infinitive)
For singular persons : base + i- :
fin-i-
For plural persons : base + iss- :
fin-iss-
Verbs with three bases.
A base for the three singular persons (
je, tu, il/elle/on)
another (generally the regular form) for the first two plural persons (
nous, vous)
another for the third plural person (
ils/elles)
By example, for the verb "to receve" :
recevoir (regular base from the infinitive :
recev-)
(je, tu, il/elle/on)
reçoi-
(nous, vous)
recev-
(ils/elles)
reçoiv-
But also the verbs :
venir (vien-/
ven-/vienn-)
tenir (tien-/
ten-/tienn-)
voir (voi-/voy-/voi-)
croire (croi-/croy-/croi-)
prendre (prend-/
pren-/prenn-)
boire (boi-/buv-/boiv-)
devoir (doi-/
dev-/doiv-)
pouvoir (peu-/
pouv-/peuv-)
vouloir (veu-/
voul-/veul-), etc.
As you may notice, the "nous" and "vous" bases are generally regular!
Very irregular verbs (just a few) : avoir, être, etc.
Enter a French infinitive and get the conjugaison.
http://www.leconjugueur.com/frconjonline.php
http://www.pomme.ualberta.ca/devoir/
Focus mainly on the bases of the "indicatif présent". They will give you the "indicatif imparfait", the "subjonctif présent" and the "impératif présent"
The infinitive will give you the "indicatif futur simple" and the "conditionnel présent"
The past participle will give you all the "composed* tenses" (*auxiliary verb avoir or être + past participle) : "indicatif passé composé", "indicatif plus-que-parfait", "indicatif futur antérieur", etc.