Trees fallen on the road is an example given in the grammar book and it says that fallen is a non-finite verb but how?Non-finites are of 3 types-ing/ed/to_ but 3rd form of verb(fallen) is not under non-finites.
Non finite verbs do not show a distinction in tense; they can not be the main verb in a clause.
In the given example: 'fallen' is a past participle, non finite verb.