Tro expresses a theory that in principle can be verified or falsified through a test:

Jeg tror, det er en god bog. (I've got reasons to believe that it's a good book - but I haven't read it)

Tro says: We'll see!

Synes expresses an opinion based upon feeling:

Jeg synes det er en god bog. (I find that it's a good book - I 've read the book and I like it).

Synes says: That's how I feel about it!
Choose: tro or synes in the drop-down-list

- Prøv lige at se: synessyntestroedetror du, at mit hår er fedtet?
- Ja, men jeg synessyntestroedetror bare, at du brugte Teen-wax freench-fry-hold. Jeg skulle lige til at sige, at jeg synessyntestroedetror, at det sad godt.
- Jeg synessyntestroedetror, du lyver.