Some French verbs do not allow their indirect objects to be replaced by pronouns; instead, the preposition must be maintained after the verb along with the indirect object.
In English, you can emphasize an important word or syllable by pronouncing it more loudly, as in "It's magNIFicent!" That's not the case in French, which uses the affective accent instead.
Most French verbs are conjugated with avoir as their auxiliary verb in compound tenses and moods, and therefore do not require agreement with their subjects. But avoir verbs do need agreement in a very specific construction: the past participle must agree with the direct object when it precedes the verb.
All pronominal verbs are être verbs in compound tenses and moods like the passé composé, which means that the past participles must agree with their subjects - at least in theory. In fact, it's not quite so straightforward.
Verbs of perception are subject to grammatical agreement in the compound tenses, but the rules are somewhat tricky - they only agree with their subjects when they precede the verb.