The conditional is, in my opinion, one of the simplest French conjugations. There is only one set of endings for all verbs, and most of them - even many which are irregular in the present tense - use their infinitives as the root. There are only about two dozen stem-changing or irregular verbs which have irregular conditional stems but take the same endings:*

acheter > achèter-   similar verbs: achever, amener, emmener, lever, promener

acquérir > acquerr-   similar verbs: conquérir, s'enquérir

appeler > appeller-   similar verbs: épeler, rappeler, renouveler

aller > ir-

avoir > aur-

courir > courr-   similar verbs: concourir, discourir, parcourir, more

devoir > devr-

envoyer > enverr-

essayer > essaier-   similar verbs: balayer, effrayer, payer

essuyer > essuier-   similar verbs: appuyer, ennuyer

être > ser-

faire > fer-

falloir > faudr-

jeter > jetter-   similar verbs: feuilleter, hoqueter, projeter, rejeter

nettoyer > nettoier-   similar verbs: employer, noyer, tutoyer, more

pleuvoir > pleuvr-

pouvoir > pourr-

savoir > saur-

tenir > tiendr-   similar verbs: maintenir, obtenir, soutenir, more

valoir > vaudr-

venir > viendr-   similar verbs: devenir, parvenir, revenir, more

voir > verr-   similar verb: revoir

vouloir > voudr-

*Notes:

   1.

The conditional stem always ends in R.
   2. The exact same verbs are irregular in the future and use the same stems.

Test on the French conditional mood

French conditional conjugations
To conjugate an -ER or -IR verb in the conditional, add the appropriate endings to the infinitive. For -RE verbs, remove the final -e and then add the conditional endings. For irregular verbs, add the endings to the irregular conditional stem. For example, here are the conditional conjugations for the regular verbs parler (to speak), finir (to finish), and vendre (to sell) and the irregular verb aller (to go):
Pronoun	Conditional ending	parler > parler-	finir > finir-	vendre > vendr-	aller > ir-
je	-ais	parlerais	finirais	vendrais	irais
tu	-ais	parlerais	finirais	vendrais	irais
il	-ait	parlerait	finirait	vendrait	irait
nous	-ions	parlerions	finirions	vendrions	irions
vous	-iez	parleriez	finiriez	vendriez	iriez
ils	-aient	parleraient	finiraient	vendraient	iraient