the word ''saw'' is the past of see, and a thing ?
on portuguese is a thing ''serra'' and the past of see is ''vi'', 
on spanish is a thing ''sierra'' and the past of see is ''vi''.

It's a thing when we treat it like a thing.  For example, we use an article or determiner (a, the, my, etc) or we put it where a subject or object should be.

When it's a verb, it comes after the subject. There's usually a time-marker as well somewhere.

So basically, we know the difference by how we use it in a sentence.