I just wrote out some thoughts on Spanish verb conjugations in order to answer a question on Reddit, and thought they might be of more general interest.
The question was how to learn Spanish verb conjugations. I recommended conjuguemos.com, as always, for verb practice. But I also summarized the different conjugations, lumping them into eight groups from a learner’s perspective.
In this effort I wasn’t careful to distinguish tense, aspect, and mood; life is too short. And of course, the longer-term challenge is knowing WHEN to use each conjugation.
The present tense takes a lot of practice because (i) it is usually the first tense you study, (ii) -ar, -er, and -ir verbs have distinct endings, and (iii) there are a lot of irregulars.
The imperfect is super-easy because (i) -er and -ir verbs have the same endings and (ii) there are only three irregulars.
The preterite, like the present, has tons of irregulars, but at least -er and -ir verbs have the same endings. I have a nice summary on my Teaching page (look for “Todo el pretérito”). My basic advice is to “divide and conquer.”
For historical reasons, the two subjunctive (present and past) conjugations are similar to the normal (“indicative”) present and the preterite, respectively. This means that once you have learned the latter it’s mostly a matter of getting used to a somewhat different set of endings. It helps that -er and -ir verbs have the same endings in the present subjunctive, and that -ar, -er, and -ir verbs ALL follow the same conjugation pattern in the past (“imperfect”) subjunctive (starting with the ellos/ellas/ustedes form of the preterite). However, the present subjunctive does have six irregulars of its own. And the imperfect subjunctive actually has two possible sets of endings (-ra and -se), though learners can just stick with the -ra set.
The future and conditional are a piece of cake because you aren’t really conjugating, you’re just sticking endings onto the infinitive, and these endings are identical for -ar, -er, and -ir verbs. Although there are a bunch of irregulars, they all evolved to simplify pronunciation, so they feel good in your mouth.
The perfect tenses with haber (like he comido) all use the same participle (the -ado/-ido thing), so once you (i) memorize a few irregular participles (like escrito) and (ii) know how to conjugate haber in the tense of your choosing, you are set.
Same for the various progressive tenses (like estoy comiendo and estaba bailando), except that here you probably already know how to conjugate estar, so all you need to learn is the present participle (the -ando/-iendo thing), which again has a few irregulars (like durmiendo and leyendo) which are predictable once you get the hang of them.
Commands build on what you already know. Mostly you use the subjunctive. The only exception is affirmative informal commands, both singular (tú) and plural (vosotros). For historical reasons, affirmative tú commands resemble the él/ella/usted form of the present tense, plus 8 irregulars, while affirmative vosotros commands simply change the -r of the infinitive for a -d, e.g. hablad ‘Speak, you guys’. A complication with commands is that object pronouns go before negative commands (No lo hagas) but glom onto the end of affirmative commands, often requiring an accent mark to maintain the normal stress position (Cómelo). Here is a summary of these complications.