Grammar is the set of rules that defines a spoken and/or written language. The beauty of grammar is that it exists at all. A language is not something designed, but built from trial and usage. That underlying rules can be identified to "explain" or "regulate" the language is therefore a non-trivial observation. Similarly, the laws of physics might be ad hoc or emerging patterns that happen to capture the bulk of something which popped out by necessity or accident, as opposed to derive from a underlying principle or will.
By construct, something which cannot be avoided in grammar is the exception, precisely because the language was not formed after its grammar, but the other way around. That makes grammar interesting but also frustrating.
We are interested in French, English, Spanish and Russian grammars.
Here we discuss the concepts which subtend all (or most) grammars.
Copula is a Latin term that means 'link', and which, in grammar, is a word or phrase that links the subject of a sentence to a subject complement (predicate).
A copula is often a verb, but unlike action verbs (e.g., "run," "eat"), a copula doesn’t describe an action but rather a state, identity, or relationship. In English, the main copula is the verb "to be". Spanish as two copulas: "ser" and "estar".
Words change depending on their function, usage, context, etc. For instance, a verb (a word purporting an action) gets "conjugated" depending on its tense, person (gender), number and possibly still other parameters. Languages where this does not happen (i.e., where verbs remain uninflected) fall into so-called isolating or analytic languages.
The adaptation of words when it applies to nouns, pronouns, and adjectives is called declension or declination. This is a central system to languages such as Russian.