Learn English – If I want to write PERIOD, is it after dot? Is dot even required

Is "hello." a sentence?

Yes.

What do you want for dinner?

Vegetable curry.

Hello! You surprised me.

Since you define a sentence as needing a verb, please realize that a sentence does not always have to express its verb. Also note that full stops are necessary in written language only. One does not punctuate spoken sentences the same way we do written sentences.

Additionally, hello is a greeting, which has a special function. When you greet people using spoken English, do you say Hello or do you only say something that has a verb, such as I bring you my greeting, which is hello or perhaps I greet you hello? Do you see what I mean? We don't always speak in fully expressed sentences. And since writing sometimes seeks to copy or represent spoken language, there will be times when something we write does not have an expressed verb, yet it will still be a sentence.

"Hello." is a good example of that. And if you want to know where the verb is, you can consider "hello" to be short for I greet you hello.

Likewise, hello! can also be an exclamation of surprise. See definition 3.

Other exclamations include

Bingo!
Dang!
Hogwash.

To me, these are sentences. If you need a verb for them to be a sentence, you can think of the sentence as

"(I say) 'Hello!'"
"(I say) 'Hogwash'."

The second line of this answer is one word: yes.

I condider "yes" in that context to be a sentence. It is answering the question that is in the first line. If you need a verb for it to be a sentence, you can consider the sentence to be

Yes, I say.

Okay? (Are you "okay" with that?)

Recommended additional reading:

Sentence Well-formedness

There are no absolute rules on how long a pause should be. As Andrew commented "it appears there may be a misunderstanding about pausing and intonation".

1. When we read a book to ourselves we may or may not make long or shot pauses, intentionally or not we might not even notice ourselves pausing.
2. When we read aloud for someone (especially if that someone is listening) we need to show the pause so that the listener can understand. Whether we do it by applying a short pause or a long one or by changing our intonation is personal, as a matter of fact, generally, it is done unintentionally. (Try to read this sentence aloud as if someone is listening)

You say that a "period and colon" is followed by approximately two seconds of wait time. Okay.

• Hello. How are you? How are your parents? - If you read this naturally (as most will) you will notice that there is almost no pause after the periods. And if there is then it's less than a second long.

Surely you can intentionally place a 2-seconds-long pause in there or wait as long as you like. You can shadow your intonation if you want. We aren't robots that have rules they obey!

• The weather was... em... what was it like? Oh, yeh, it was raining!

In this sentence you can add very long pauses if you want to show that the person was trying to remember. You can pause for more than 2 or 3 seconds in the first part and in the second part too. You will, probably, intentionally show emotions and intonation.

Conclusion: Opinion on how long a pause should be is personal. Everything depends on the person, text, and his ability to read aloud or/and speak. Pause and intonation are different things.