Question for Spanish speakers

When starting a sentence with a person “grandpa” or “he” for example:

What is the rule on when to start the sentence with a “a” versus when not to?

Ex: mi abuelo tiene un regalo para ella

is okay

But

A el le encanta ir al zoologico

Requires a “a” first.

I feel like I’ll get lots of explanations specific to these two examples, but I need to know the overall rule as to when it is or isn’t necessary please.

Thanks!