Why is I am reached incorrect while I am done is correct?
Ex: I reached office at 9am today is correct while I am reached at 9am is incorrect.
But I am done with this project is correct. Can someone please explain me the difference here?
did and reached are both past forms: they can be used to make a simple past sentence:
I reached the office
I did my homework
done and reached are both past participle forms. You can use a past participle to form one of the three perfect tenses:
I had reached the office...
I have reached the office...
I will have reached the office...
You can also use a past participle of a transitive verb to form a passive voice sentence:
I reach him - active voice
He is reached - passive voice
He is reached by me - passive voice
Note that the object him of the active voice sentence becomes the patient he in the passive voice sentence, and the subject I either disappears or becomes the agent attached with by
Looking at your two sentences:
I reached the office at 9am
I am reached at 9am
In the first sentence, reached is the past form of reach. The sentence is past simple, active mood, with I as the subject - I reached the office.
In the second sentence, reached is a past participle form of reach. The sentence is simple present, passive mood, with I as the patient- something reached me. It sounds strange to use simple present to describe an event that happened in the past, but this technique is a perfectly acceptable narrative style.
Both sentences are grammatically correct, but the meaning of the second version is probably not what you intended.
If you look at the Cambridge Dictionary entry for done, you will find that it can be both a verb-form and an adjective. The reason that you can use I am done without switching to passive voice is that done can be used as an adjective. Here is a definition of the adjective form:
If something is done, or you are done with it, it is finished, or you have finished doing, using it, etc.
If you look in the same dictionary for reached, you will find that there is no adjective form.