Couple of beginner questions

I've done a number of sites in html and am now venturing into php. Can I create a page in html and insert php code that will work? (for example, take an existing page and insert a date command) Can I create a page with the php extension that contains only contains html and no php? If so are there advantages/disadvantages? Can I mix and match file formats (php/html) in a single site? Thanks for any input. Gary