Starting sentences with and, but, however or because
The idea that a sentence should not begin with coordinating conjunctions such as and and but has no grammatical foundation. The main issue is stylistic: they can become tiresome if used too often at the start of sentences.
Starting a sentence with because is no problem when it is the subordinator for a dependent clause that precedes the main clause:
The same goes for other subordinators such as conditional if, which often leads a sentence setting out the precondition for an argument.
However can be used at the beginning of a sentence (or later), as an indefinite adverb or conjunction. It is typically followed by an adjective or adverb:
However can also be used as a contrastive adverb at the start of a sentence, set off with a comma:
In formal writing, the 2 sentences can be combined, but the boundary should be marked with a semicolon, and a comma after however: