When I hear someone say, “I was never any good at grammar,” I can only think that the person was never taught basic grammar in elementary school.
Grammar is a branch of linguistics, which is the science of language.
This site is not concerned with linguistics. All it’s concerned with is the everyday speech of ordinary English speakers.
People who were born into an English-speaking family already know the basic patterns of English.
Different families speak different forms of English called dialects. Different dialects have distinct pronunciations, sentence-constructions, and vocabulary that speakers of other dialects may not easily understand.
Beginning in the 1960s, English teachers were discouraged from teaching grammar as a separate topic. The theory from the experts was that students would pick it up from reading and writing.
One result of the theory is a generation of students and teachers who have a very shaky grasp of the parts of speech and the parts of a sentence.
Another result is a population of high school and college graduates who are unable to speak and write with grammatical correctness.
An important function of US public education is to teach all children a form of Standard American English they can use in addition to their home dialects.
Anyone who can learn to drive a car or master the rules of basketball can learn enough grammar to avoid the most glaring grammatical errors. The place to begin is with the Parts of Speech.