Not a sentence. Delete?

Descriptively it is definitely a sentence.

How come? It seems like a part of complex sentence, i.e. "He had no one to help him with money, because all his friends were poor, too".

"All his friends were poor, too" is a sentence.
"Because all his friends..." is also a sentence.
I can't see much difference between them.

The sentence above is obviously an answer.
But the other sentence without the word "because" is also a part of a text.
We usually say sentences one after another.
I think we rarely say sole sentences (e.g.: Come here.) .

The following definition of sentence is in Cambridge dictionary:
a group of words, usually containing a verb, that expresses a thought in the form of a statement, question, instruction, or exclamation and starts with a capital letter when written: ...

Many sentences on Tatoeba don't really express a thought. However, this one does.

#413731
#6586950
#3990110

From that point of view, "To write with a pencil" is a sentence, too. It might be an answer to "My pen doesn't work. What should I do?"

What is a sentence in fact?

#31248 (A little louder.) is not a sentence. It would be a sentence in the following way: Speak a little louder.
(The next one: A little more.)
You should've delete about two hundred thousand sentences.

I can say "It's red" is not a sentence either, since I don't know what it is about.

No answer. Deleted.