Tense for a Stopped Clock – Sentence Construction and Tense Usage

Right, as you say, when we use the past continuous tense there is usually some context that makes it clear to the reader / listener when the event was happening. Your question seems to be mostly about stative verbs (1) to be; and (2) to see.

(1) Be

Verbs that are not typically used in continuous tenses are called stative. Examples are see, be, want, hear. So we say "I was in bed..." not "I was being...". There are some cases when stative verbs like "be" are used in a continuous tense and these cases tend to change the meaning of the verb.

(2) See

As to the correctness of "I saw a very big bird at 10am this morning", this will depend on the context. It is a grammatically correct sentence; it appropriately expresses the idea that at 10am a bird appeared in view.

However, if you wanted to express some kind of continuous observation then you would need a different verb, not see (which is stative). E.g. you might say, "when someone knocked on the door at 10am I was watching a big bird".

As others have indicated, you don't want to say:

Oh I was almost to forget to mention...

(For one thing, the was is inappropriate there.) Instead, you'd want to use one of these:

Oh, I almost forgot to mention...
Oh, I forgot to mention...

It's interesting how the word almost is optional in this context.

Let's say I meant to tell you that you got some mail. As you're walking out the door, I say:

Wait! I almost forgot to mention: You got a package today.

I almost forgot to mention it – but then I remembered in the nick of time!

Alternatively, I can say:

Wait! I forgot to mention: You got a package today.

This means: I had forgot to mention it (until just now). But now I'll mention it.

At least in AmE, I hear and use both of these. People don't seem to get hung up on whether the almost should be included because, with or without it, the sentence can sound correct.