That depends upon the sentences. But there is no prohibition against those words at the beginning of sentences.
Student or Learner
I have heard that it is not elegant to start sentences with the words: "because", "since", "but", "and"?
Is that true.
But it would not be elegant to start a conversation with these words. )))
***** NOT A TEACHER *****
I have read that if one understands the rules, then one can occasionally break them. For example, professional writers presumably know the rules, so they feel free to break them.
Learners should probably refrain from starting sentences with those words. Furthermore, in formal contexts (at the university, for example), it might be a good idea to avoid such sentences, too.
As you read more and more English, you will find, however, that many native speakers occasionally start sentences with those words -- for a more dramatic or emphatic effect:
"Despite her parents' objections, Mona said that she would marry James, and she did!"
"Despite her parents' objections, Mona said that she would marry James. And she did!"