I found this sentence in this link: http://watanoc.com/post-1608-gozira So there's a part where it says:アメリカの映画だと思っていました but why to use the て-form instead of the ます-form? The meaning wouldn't be the same if I use the ます-form?
There is a clear difference in meaning.
means "(I) thought". You thought (or felt) something momentarily in the past (including the immediate past "just now").
means "(I) have/had been thinking/feeling". The thinking lasted for some time. That time period could be quite short or it could be very long such as multiple years or even your whole life so far.
In very informal conversations, however, some native speakers might use 「思いました」 instead of 「思っていました」, but not the other way around.