¿Cuál es la diferencia entre I lost my keys. y I have lost my keys. I never understood when you Americans use the verb "have" to indicate past. ?Puedes dar oraciones como ejemplo.

In general, they can be used interchangeably and you'll be understood perfectly well.

Here, one could take "I have lost my keys" to mean in general this has happened to them once before.

Example: Who here has lost their keys?

A) I lost my keys (implying a recent or specific event)
B) I have lost my keys (in general this has happened once before / nonspecific)

Also note that technically "have" is used to produce the present-perfect tense which reflects an event that occurred in the past and has continued up to and is continuing in the present.

For example: "He has worked there a lot."

This implies he may still work there. Whereas "he worked there" is pretty clear that he no longer does.

Sorry for such a long response. Basically this is an example with a very fine nuance to it. But suffice it to say that 99% of people will take them both to mean the same thing.