na-adjectives in different tenses. これは真面目だった人です。 I need some help with basic grammar. What is the difference between these sentences: > 1. > 2. > 3. > Thank you in advance!

I'm only learning myself so I'm not sure, but I think all sentences are correct, since "/" are both the present/past prenominal forms of "", and I don't think these collide in any way with the / copula at the end of the sentence.

1. This _is_ a person who _used to be_ serious.

2. This _was_ a serious person.

3. This _was_ a person who _used to be_ serious (but that changed at some point).