conjunctions - Should we not start sentences with "And" , "But" ,"So" and "Because"?

Personally, I use “but” and “so” to start sentences, but not “and” and “because”.

Today I was talking to a friend of my brother on this matter, who is in 6th Grade. He takes tuition from a private tutor who told him that in early days using “and”, “but”, “so” to start a sentence was okay, but now-a-days it is becoming obsolete and that we should not use them to start a sentence. He is a reputed teacher here and he keeps track of current affairs. (He is the one who told me, when I was his student, that “will” is being used after “I” now-a-days and I followed the advice.) So I could not deny this information. Tried to find in Google about this but no source made me think it is trustworthy. So I would like to know this in StackExchange,

What is the actual rule and is this rule being changed?

PS- I don't use “because” to start sentences but I always had a doubt. So I added it!

Answer

There is a frequently taught "rule" that sentences should not start with any of the coordinating conjunctions, i.e. and, or, but, yet, so, for, and nor. There is no actual grammatical rule as such, however, simply because such a rule doesn't make any sense. In fact, even the most ardent prescriptivists are likely to agree that starting a sentence with a coordinating conjunction is perfectly fine.

However, there is a reason that this so-called rule is taught. The goal is to avoid sentence fragments such as:

*He went to the store. And bought milk.

It is much easier to forbid starting sentences with a coordinating conjunction than to explain exactly how sentence fragments work, and this is how this "rule" came about. A perfectly fine sentence that does begin with a coordinating conjunction is, for example,

He went to the store. And having seen the milk, he bought it.

The word because is liable to similar errors; for example,

*He went to the store. Because he was hungry.

Like the coordinating conjunctions, it is perfectly fine to begin sentences with because, so long as you keep in mind that the goal is to avoid fragments. For example, the following sentence is perfectly acceptable.

Because he was hungry, he went to the store.