I’m afraid you’d have to learn the conjugation tables (tables de conjugaison).

Fortunatly, many verbs share the same conjugation. They are divided in 3 groups:

• The first group contains the verb which end in -er and are all conjugated the same way.
• The second group contains many of the verbs that end in -ir and are also conjugated the same way.
• The third group contains… All the verbs that don’t belong to the 1st and 2nd group.

Even for the first group (which is commonly considered the easiest to conjugated), there are 6 persons to learn for each tense.
For example the verb manger (to eat) which belongs to the 1st group gives in the présent simple de l’indicatif:

• Je mange
• Tu manges
• Il mange
• Nous mangeons
• Vous mangez
• Ils mangent

The verb finir (to finish) belongs to the 2nd group and gives:

• Je finis
• Tu finis
• Il finit
• Nous finissons
• Vous finissez
• Ils finissent

While there are common patterns (1st plural person ends with ons, 2nd plural ends with ez, etc.), there are also subtilties that makes learning conjugation a nightmare for French kids, and I don’t want to imagine what it is for an adult learning French!

… And that’s only for the most simple tense.

You can apply some of the same principle to ease your study of French verb, but the greater variety of inflexional patterns and of inflected forms in general makes the exercice a lot more complex.

I’m also going to ignore orthographic distinctions that aren’t reflected in the pronunciation (like vous mang-ez vs. nous mange-ons, or il vint vs il vînt) for this answer, to avoid a further layer of complexity.

First, a note: The three form template of the Germanic verbs doesn’t list every possible form of the verb, just enough to let you generate the rest through regular transformations: Knowing weten – wist – geweten in Dutch let you generate "ik weet", "ze wisten" or "ik had geweten", for example. The same principle can be applied to French conjugation: knowing one of the past simple forms of aller, for example, lets you generate all the other past simple and past subjunctive forms: j’all-ai -> tu all-as, il all-at, nous all-âmes, vous all-âtes, elles all-èrent, j’all-asse, tu all-asses, il all-ât, and so on.

Sadly, each of the three conjugation classes will have its own set of rules, and its own number of needed stem to generate its whole inflexional pattern.

1st group (regular -er infinitives)

There’s two types of 1st group verb, each with its own regular patter:

• Single stem verbs like parler, manger, couper or jouer. Those are the simplest verbs in the language, once you know one form, you have enough information to generate all the other ones. Simply remove the -er suffix and add the appropriate one for the tense/aspect/mood+person combo you need.

• Double stem verbs like nettoyer, essuyer, jeter, enlever. Those alternate between one form used in the singular and 3rd person plural of the indicative present and the subjunctive present, as well as the future and conditional (je nettoie, que je nettoie, je nettoierai, je jette, que je jette, je jetterai) and another used for the rest of the conjugation (nous nettoyons, en nettoyant, ils nettoyèrent, nous jetons, en jetant, elles jetèrent)

This means that all you need to memorize for -er verbs is the infinitive and one present singular form. If they use the same stem without pronunciation change (manger – je mange), they’re a single stem verb, if the pronunciation changes (lever with /ə/, je lève with /ɛ/), they’re a two stem verb. In either case, this gives you enough info to conjugate them correctly.

There’s two irregular -er verbs, aller (a 3rd group verb in disguise) and envoyer, whose future/conditional stem is enverr- instead of the expected envoier-.

2nd group (regular -ir infinitives)

Those are simple, they regularly alternate between two stems: one in -/i/ used in the infinitive, the past participle, the indicative present singular, the past simple, the past subjunctive, the future and the conditional (fini-r, je fini-s, je fini-s, que je fini-sse*, je fini-rai, je fini-rais, fini) and one in -/is/ used in the plural of the indicative present, the subjunctive present, the imperfect and the present participle (nous finiss-ons, que je finiss-e, je finiss-ais, finiss-ant)

(*) que je finisse looks like the second, longer stem, but remember that -sse is the 1st person singular past subjunctive suffix (que je fisse, que j’envoyasse, que je crusse, que je vinsse, etc)

It’s thus enough to memorise two forms those verbs, one in -i and one in -is, to know how they conjugate and to distinguish them from the irregular -ir infinitives like souffrir. For example, finir – je finissais.

3rd group (the irregular verb hodgepodge)

Here comes the real locus of complexity. The third group is really a composite of several verbs classes, sometimes themselves further split into smaller variants. Some of their forms, typically the future/conditional and the past participle, can be completely unpredictable from the infinitive. That’s where a Germanic-style stem based approach can be useful.

Let’s review some patterns:

• The indicative present is the most complex tense in the language. It can involve up to 4 unpredictable stems (je vais, tu va-s/elle va-t, nous all-ons/vous all-ez, ils vont). Nonetheless, we can see a pattern: except for avoir, être and aller (and pouvoir in some high register forms like je puis/puis-je), all the person of the singular share a stem: je sai-s, tu sai-s, il sai-t; j’offre, tu offre-s, il offre. Except for dire, the first and second persons of the plural share a stem: nous av-ons, vous av-ez; nous buv-ons, vous buv-ez.

• All the persons of the subjunctive present share the same stem: je dis-e, tu dis-es, elle dis-e, nous dis-ions, vous dis-iez, ils dis-ent. This stem tends to be identical to that of the 3rd person plural of the indicative present (ils boiv-ent <> qu’ils boiv-ent, ils meur-ent <> que je meur-e), but there are too many exceptions to make this a rule (ils savent <> qu’ils sachent, elles font <> qu’elles fassent)

• All the persons of the imperfect share a stem: je pren-ais, tu pren-ais, elle pren-ait, nous pren-ions, vous pren-iez, elles pren-aient. With the exception of savoir (sachant), the present participle always uses the same stem as the imperfect (pren-ant)

• The future and the conditional share a stem, that is used in all persons. There is no exception.

• The past simple and the past subjunctive share a stem, that is used in all persons: je vin-s, tu vin-s, il vin-t, nous vîn-mes, vous vîn-tes, ils vin-rent, que je vin-sse, que tu vin-sse, qu’elle vîn-t, que nous vin-ssions, que vous vin-ssiez, qu’elles vin-ssent. (the circumflex is linked to the presence of a /s/ in the old form of the suffixes -smes, -stes and -st)

• The past participle is often irregular and has to be learned on its own.

In all, this means that (except for the extremely irregular être, avoir, pouvoir, savoir, aller and dire, all of them high frequency verbs you should learn on their own), you "only" to memorize 9/10 forms for each 3rd group verbs: the infinitive, 3 forms of the indicative present (singular, nous/vous, ils/elles), the imperfect stem, the subjunctive present stem, the future/conditional stem, the past simple stem and the past participle (ideally in both its masculine and feminine forms).

For example:

prendre, je prend-s, nous pren-ons, ils prenn-ent, qu’ils prenn-ent, je pren-ais, je prendr-ai, je pri-s, pris.e

boire, je boi-s, nous buv-ons, ils boiv-ent, qu’ils boiv-ent, je buv-ais, je buvr-ai, je bu-s, bu.e

souffrir, je souffr-s, nous souffr-ons, ils souffr-ent, qu’ils souffr-ent, je souffr-ais, je souffrir-ai, je souffri-s, souffert.e

dissoudre, je dissou-s, nous disolv-ons, ils dissolv-ent, qu’ils dissolv-ent, je dissolv-ais, je dissoudr-ai, je dissolu-s, dissolu.e

As you can see, there often is overlap between each stem, but not in a regular pattern from verb to verb.