I think it's the worst film I've ever seen.

[Maybo]

I have a question about tenses.

Example from Ch.90:

The film was very bad. I think it's the worst film I've ever seen.

(Essential Grammar in Use 3rd edition by Raymond Murphy)

Why do we use "it is" but not "it was"? If the speaker say "it's....", why doesn't he also say "The film is"?

 

[jutfrank]

The speaker says it is because it currently ranks as the worst film he's ever seen. When he says the film was, he's really talking about a past experience (i.e., that particular viewing) of the film.

 

[Maybo]

I see. The sample sentence in OP emphasises the current act of ranking, while the following underlined part emphasises that the act of seeing or ranking happened in the past:

John: How was your birthday party yesterday?
May: Great! My mother bought me a cake. It was the biggest cake I've ever seen.
https://www.usingenglish.com/forum/threads/287439-that-s-the-biggest-cake

So if I say "I think it was the worst film I've ever seen", that sentence emphasises that the speakers ranked the film in the past or the act of seeing the film happened in the past.

Did I get the concepts right?

 

[jutfrank]

That's basically right, yes.