Words at the beginnings of sentences
Most of these beliefs are wrong. Indeed, I cannot think of a single word in English that can NEVER be used first in the sentence. It all depends on the grammar and usage in the case in point - and on punctuating the sentence properly.
And, for example, is said to be a word you cannot use at the beginning of a sentence. There is a logic here: and is a co-ordinating conjunction (a joining word), and there should logically be at least two things to join, one before it and one after it. At the beginning of a sentence, there is, properly speaking, nothing before it. This is all very well, and indeed sensible. But it is not the whole truth. Language is not merely logical; we use it for many purposes, such as to sound good, or to move our hearers. To make a strict rule that and should never be used to start a sentence ignores the fact that in the great masterpiece of English style, the Authorised, or 'King James', Version of the Bible, a substantial proportion of the sentences start 'And...'. An initial 'And' after the speaker has drawn breath can make a speech very effective, as many politicians know. If you are writing with exclusive attention to logic, don't start your sentences with 'And' (or 'But', which is also a co-ordinating conjunction). If you are deliberately trying to be effective in your style, you may do so. (If you know that your teacher or other reader has a prejudice against using and to begin a sentence, of course, don't do it. Good writers (and sensible students) always think of their readers.)
The case of 'because' is different. Because is a subordinating conjunction. Properly used, it links a Main Clause, which gives the most important fact or idea given in the sentence, with a Subordinate Clause which explains the cause of the main fact or idea. The usual order in a sentence is
- [most important idea]Main Clause, because [explanation]Subordinate Clause.
But there is nothing to stop you changing this order for effect - perhaps because you want to stress the explanation rather than the consequence. The order can be
- Because [explanation]Subordinate Clause, [most important idea]Main Clause.
(Note the comma between the clauses.) This is entirely legitimate. What you should never do is write a sentence that starts with because, gives the [explanation]Subordinate Clause -, and then fail to complete the sentence with a Main Clause. The line Because it is there. is not a sentence. It feels incomplete because it is incomplete. It only gives a reason, and not the point for which it is the reason. It is better grammar and sense to say People want to climb Mount Everest because it is there; or Because it is there, I can see it.
The same applies to other subordinating conjunctions. The word while, for example, in one use means roughly the same as because, and should be handled the same way. But I have seen several students write a 'sentence', beginning with a capital letter and ending with a full stop, which contains nothing but a Clause starting with While. This is a Subordinate Clause, and should not stand on its own in formal English writing.
Yet another problem occurs with 'However' and 'Therefore'. These are words that can legitimately be used to begin sentences. They cannot be used to join sentences, as in He was old, however he was not wise. (This should be He was old. However he was not wise.) 'However' and 'Therefore' are sentence adverbs, not conjunctions. They mark the way the writer is thinking. They do not indicate a logical link between one idea and the next. So use them, with a capital letter, after a full stop to indicate a new sentence. You can, however, use them like this - parenthetically, as an aside. It is, therefore, not possible to say that these words must always follow a full stop.