Firstly, let’s focus on forms. Latin has four subjunctive tenses, one present and three past. For the time being, you’ll be using the present one only for jussive subjunctives, but you’ll meet all four in the subordinate clauses.
The present subjunctive has endings very similar to the regular present endings, active and passive. They are -m, -s, -t, -mus, -tis, and -nt for the active, and -r, -ris, -tur, -mur, -mini, -ntur for the passive. You’ll also need the verb’s present stem.
Now for the trick: between these two things – stem and ending – you put a particular vowel or vowel combination depending on the verb’s conjugation. This is confusing, unless you invent a handy mnemonic, like we eat a ciabatta: the vowels you need are e, ea, a, and ia to give (e.g.) amem, timeas, regatur, sentiamus. See how it works? A mnemonic is a great way of helping to differentiate between present indicative and subjunctive.
The imperfect subjunctive forms are the easiest subjunctives to form and recognise. Why? Because the verb’s infinitive will always be there in full, just before the subjunctive endings we used above: amarem, timeres, regeremur, sentirent.