I was most impressed with the trees growing out of the water in the middle of the pond.
(Q1: What is the function of 'most' in this sentence? Can we use 'the' before 'most'? 2. Can we substitute 'from' for 'out of'? Q3: Is ' impressed' a past participle?)
1. "most" is an adverb here, modifying "was impressed" -- and no, don't use "the" here.
2. Yes, either preposition works.
3. Yes, it's a past participle, but since it's being used here with the auxiliary verb "was," it's creating a past tense of the main verb of the sentence.