“Grammar as rules” how would you describe it with example?

grammatical rule - a linguistic rule for the syntax of grammatical utterances

We'll use who or whom for an example; which one is proper;

Use the he/him method to decide which word is correct.

he = who

him = whom

He wrote the letter. Therefore, who is correct.

For who/whom should I vote?

Should I vote for him? Therefore, whom is correct.

We all know who/whom pulled that prank.

Grammar--a set of plans for the machinery that powers all language

A rule: verbs have voice, tense and number and the voice changes upon conditions.

Indicative voice is for statements which are true and written as declarative sentences.

Imperative voice is for commands.

Subjunctive voice is for statements which are doubtful, hypothetical or not true.

If he was taller, he could play basketball. Because he's not taller and he doesn't play basketball the verb mood changes from the indicative (was) to the subjunctive (were).

This rule has dropped from ordinary usuage because rules change with usuage. But, it should still be used with conditional clauses such as: If he were taller.

Grammar 'rules' oil the machinery of language so that what is meant is what is conveyed. The rules may change as meaning becomes accepted. This is one rule which one should follow simply because it displays a knowledge of many rules. Meaning that knowing the rules allows you to break them, but not knowing them makes for mistakes.