Singular must go with singular and vice versa for plural.
However, Grammarly mentioned one sentence as correct and other as incorrect. Please check the below two sentences.
- I usually post questions on this website.
- I usually posts questions on this website.
Subject: I. (Singular) Verb: post (this also need to be singular), so posts
However, "posts" is shown as incorrect on Grammarly. Why post? It is plural.