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. これは真面目だった人です。
>   2. これは真面目な人でした。
>   3. これは真面目だった人でした。
>

Thank you in advance!




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

1.これは真面目だった人です。 This _is_ a person who _used to be_ serious.

2.これは真面目な人でした。 This _was_ a serious person.

3.これは真面目だった人でした。 This _was_ a person who _used to be_ serious (but that
changed at some point).




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

1.これは真面目だった人です。 This _is_ a person who _used to be_ serious.

2.これは真面目な人でした。 This _was_ a serious person.

3.これは真面目だった人でした。 This _was_ a person who _used to be_ serious (but that
changed at some point).

