Hello Lenka.
13) have sewn is correct. It's the present perfect: HAVE + past participle.
2) Yes, it's true. "true" is an adjective;
Yes, it's the truth. "truth" is a noun. It takes an article/determiner, "the/a".
6) *"anyone
can't do it" is incorrect. Try, "no one
can do it." Negate the subject, not the verb:
Affirmative: anyone can / any one person can
Negative: no one can / no one person can
8) "It is a very nicely flowered garden."
Sounds lovely to me. It's English.
EX: "He did it easily."
"easily" functions as an adverb. It tells us how:
EX: How did he do it?
EX: Easily is how he did it.
It’s a very nice story.
"very" modifies "nice", an adjective, so it functions as an adverb in that sentence.