"Whether" is nearly always replaceable by "if", for embedded questions. So
I don't know whether Peter will come to school.
and
I don't know if Peter will come to school.
have exactly the same meaning: I don't know if it is true or false that Peter will come to school.
Your other sentence,
I don't know that Peter will come to school.
is grammatical, but in principle has a different meaning (though that meaning is unlikely with a first person subject).
If we switch to a third person subject:
Jane doesn't know if/whether Peter will come to school.
means that she doesn't know the truth of the question. But
Jane doesn't know that Peter will come to school.
implies that it is a fact (known to the speaker) that Peter will come to school, but that Jane doesn't know that fact.
Clearly, this distinction is unlikely for a first person subject, because the subject is the speaker. For this reason, I think, you do hear
I don't know that Peter will come to school.
with the same meaning as if/whether; but I would say this is very colloquial.
attain X = Work to get or reach X. X is usually a goal or milestone.
achieve X = Work to become X. X is usually a status or something having to do with recognition.
obtain X = get X.
acquire X = get an X that was not had before or was needed.
gain X = get an additional X or add X to something we have.
Best Answer
To "come by" means "to come to have" or "to come to get" here:
Here, it means "to arise from" (i.e. to come to be as a result of):
Come conveys the idea of events-as-they-unfold. The statement and questions in the top set speak and ask about the circumstances that led up to the thing. For that reason, to my ear, Where...come by...? is slightly unidiomatic whereas *How...come by...? is idiomatic.
These are actually different verb phrases, and would be spoken with different parsing rhythms and intonation patterns.
Success does not come {pause} by brains alone.
How did you {come by} that haircut? { } = spoken as a unit without a syntactic pause between.