Which is correct sentence structure?

A sentence follows Subject + Verb + Object word order. He (subject) obtained (verb) his degree (object).

Is he don’t know correct?

Yes, it is grammatically correct to say “He doesn’t know”. That is the format you should use. The use of don’t in that sentence is grammatically incorrect. It is a sub-standard slang usage.

When use came or come?

The past tense or simple past tense of come is came “I have come” is the present perfect tense which is formed using the present form of ‘to be’ + the verb’s conjugation that is in the past form.

Did not come or came?

It is imperative for the first form of the verb to be used in juxtaposition to “did”. And that is why, “didn’t come” is correct, which is clearly the first form of the verb and not the third, which is also the same, the three forms being: come-came-come.

Which sentence is correct he doesn’t know how to speak in English?

He doesn’t know how to speak in English is right Answer. In Present Indefinite Tense we have two Helping verbs Do and Does. DO we use with I, You, They and We. Does we use with He, She and it. It is clear now that as per this rule we use Does with He. So right sentence is He doesn’t know how to speak in English.

Is it correct to say I don’t know when he will come?

Overall, the first sentence, “I don’t know when he will come” is grammatically correct, and is a safer bet than the latter. HELLO! AS I SEE IT BOTH OF THEM ARE WRONG.

Is it correct to say “I don’t really know”?

“I really don’t know” emphasizes the negative aspect. This word order would be used in a situation where it has been insinuated that the speaker does know something. “I don’t know, really” emphasizes the entire sentence. This would be used if there had been so “I don’t really know”? They are all correct.

Is “I didn’t know” correct grammar?

“I didn’t know” is correct. Always. “I didn’t know” is wrong. This is a very common mistake which most common laymen make who don’t have a strong knowledge of English Grammar rules. Look at it in this way: didn’t = did + not.