The words however and therefore, when used at the beginnings of sentences (or in similar uses in the middle of sentences), are not technically conjunctions, or joining words: they are sentence adverbs. You can ignore the jargon: just remember that they are properly used with full stops (or punctuation marks of similar weight, colons and semicolons). Don't end one sentence, and then use a comma and therefore or however to add a second sentence. True joining words (conjunctions, to be technical) link one idea with another, and either have a comma or no punctuation at all.
- It may be worth AWE pointing out a distinction of meaning:
- however (adverb) indicates a difference with what was written before;
- although/though/even though (conjunctions) most commonly refer to what follows.
In punctuation, though etc usually have a comma. However normally comes after a full stop. If you write , however or , therefore you should normally have a second comma, however, or , therefore,.
- The above does not apply when however is used to qualify
an adjective, such as "however unlikely" or the full clause such as "however unlikely it may seem". In such cases, however is a subordinating conjunction, and should be used with commas around it and the element which it is qualifying.
See also words at the beginnings of sentences.