While there's nothing grammatically wrong with "I am invited to the party", and I would understand what you intended to say, it's not the way a native speaker would phrase this. If I was discussing my invitation to a party that hasn't happened yet, I would say:
I've been invited to the party.
(I have sounds far too formal, so I've gone with the contraction I've here.)
As for your second example:
I was invited to the party.
I think you're correct that no implication of time is made here. The invitation clearly happened in the past, but the party could have been a week or a year ago, or later tonight, or two weeks from now (or even now!) Some possibilities:
I was invited to Mark's birthday party last year, but then they cancelled it at the last minute!
I was invited to the party next week, but I've got the kids that weekend so I might not be able to go.
Why do I have to leave? I was invited to this party! I have the invitation right here, see?
So while the invitation took place in the past, in this case I would say that the actual party could be at any time; context will make that clear.
Both sets of sentences are correct, the difference between them is the meaning.
Now this difference is a little hard to explain, but I will do my best.
The verb 'to have' conjugates in the present as 'have' or 'has':
- I, you, we, they, you all = Have
- he, she, it = Has
In the past, it conjugates as 'had' for everyone.
Now when it comes to questions starting with 'to have', asking if someone has done something, they are asking about one of two situations, each with it's own meaning.
When you use 'had' (past tense), then you are asking about any time up to a specific moment in the past.
Had you run a marathon?
Meaning "At any time up until that moment in the past, did you run a marathon?", or "Did you run a marathon by then?"
When you use 'have' or 'have', you're asking about any moment up until right now.
Have you run a marathon?
Meaning "At any time up until now, did you run a marathon?", or "Did you run a marathon by now?"
The difference between the two is the limit of the time asked about. When you ask using the past tense of 'to have', then the limit is referring to a specific moment in the past. When you ask using the present tense of 'to have', then the limit is this very moment, now.
- Have, has = "up until now"
- Had = "up until then"
I hope that helps!
Best Answer
If you are listing items in a timeline by saying the time, then the event, you can use the present tense (I expanded on your example a bit):
You can also use the past tense and it will still make sense.
Again, the above only works if you are reading a list off to someone.
If you are simply saying what John did in a sentence, use the past tense: