In ex 1 I think hiding and hidden would have slightly different meanings. Who knows what exciting hints may be hiding in your tree. - this has an active connotation to me (the hints are hiding) Who knows what exciting hints may be hidden in your tree - this has a passive connotation to me (the hints may have been hidden by something/ someone or the hints are hidden by/behind something like leaves or branches) In ex 2, my understanding would be much the same. I'm leaning over the table - active. I'm leaned over the table - this has a more passive sense. A clearer example could be: The ladder was left leaned against the wall. (You could also say the ladder was left leaning against the wall)

The present participle of any verb can be used as an adjective: the flying bird the swimming boy the singing girl Adjectives become "predicate adjectives" when they are tied to the subject by a linking verb: the bird IS flying the boy IS swimming the girl IS singing You can do the same with adjectives that are not participles: "the blue bird" and "the bird is blue". In example 1, "exciting" and "hiding" are both used as adjectives. You can even switch them and you still would have a perfectly grammatical sentence, although it wouldn't make a lot of sense: "Who knows what hiding hints may be exciting in your tree". Past participles, such as "leaned" (also "leant") (the p.p. of "lean") can also be used as adjectives. However, using them for this purpose introduces "passive voice". So, if you say "I'm leaned over the table" it sounds like someone else may have leaned you. It's better to say "I'm leaning" because that expresses the fact that you yourself are the one who is doing the leaning.

In example 1, you can use "hidden" or "hiding" interchangeably. "Hiding" is expressed as if the hints were sentient and could actively hide, but idiomatically it's fine. In example 2, you cannot use "leaned". "Leant" sounds a bit better but still not quite right; "bent" would be fine ("I'm bent over the table"). "I'm leaning over the table" maybe expresses the idea that you are actively leaning over the table, whereas if you were asleep or unconscious with your torso on the table, you would not be leaning over it, but you would be bent over it.

Definitely, “I leaned over the table” would be past past. Past tense doesn’t have to mean yesterday or five years ago. It can be two seconds ago, even. “I’m leaning over the table is happening now and shows some progression in time. But, people will sometimes use present tense, or progressive present tense, in talking or writing to get the listener or reader in the mood of “right now”. For example, someone could be talking about something that happened yesterday in the kitchen. The person talking is using past tense because it’s about what happened yesterday. Then suddenly the talker (or writer) will go into present tense for the sake of the mood.