Learn Farsi: چند سال قبل اینجا بودم.

When ghabl is placed after a period of time (like chand saal / a few years), it translates to "ago" instead of "before", and we drop the az. If you wanted to say "before a few years", you would use ghabl az, but to say "a few years ago", you simply say chand saal ghabl.

In English we say "a few years" (plural), but saal (year) doesn't have a plural ending here. Why is that?

In Farsi, whenever you use a number or a quantity word like chand (a few), the noun that follows it must stay singular. It is a strict rule, so it will always be chand saal, never chand saal-ha.

I learned that chand is a question word meaning "how many". How can I tell it means "a few" in this sentence?

Chand can mean both "how many" and "a few" or "several". You can tell which one it is from the context and the grammar. Since this sentence is a statement ending with a period rather than a question mark, chand acts as a quantity word meaning "a few".

Could I use the word pish instead of ghabl to mean "ago" here?

Yes, absolutely! Chand saal pish is a very common alternative and means the exact same thing: "a few years ago". Both ghabl and pish work perfectly in this structure without .