Present Perfect Simple Tense

I've washed the dog. (replace washed with walked or fed and see what will happen)

Do we know when I washed the dog? -No, we don't.

But, how do we know the dog is washed? - It's not dirty.

They've bought the car. (replace bought with painted or sold and see what will happen)

Do we know when they bought the car? - No, we don't.

But, how do we know they bought it? - It's new.

 

He's lost a book. (replace lost with borrowed or signed and see what will happen)

Do we know when he lost the book? - No, we don't.

But, how do we know it's lost? - He no longer has it.

We are familiar with HAVE (short form 've) / HAS (short form 's) and the past simple of regular (washed, walked, painted, etc.) and irregular verbs (buy - bought, lose - lost, etc).

Now, take a look at the following:

I've been to Egypt. 

She's seen the Pyramids too.

In these sentences we have the same situation we talked about before - we don't know when something happened but we can see the result (photographs, souvenirs or things like that). But, what are been and seen? They are past participles of the verbs we are familiar with - be and see. We use past participles with HAVE / HAS, and not past simple forms, for this new tense that connects the past and the present.  

Let's take a look at the list of irregular verbs again. As you can see, past participles are in the third column. Some are the same as past forms, the others are completely different. That's why my advice for you would be to learn those with the same form first and leave the different ones for later. Hope it helps.