_By the way_, I met a friend of yours today.
We cannot, _however_, join you at once.
He called, _it seems_, while we were gone.
In these sentences such words as, _however_, and such phrases as, _by
the way_, and, _it seems_, are used independently,--in parenthesis, as
it were; that is, they are just thrown into the sentences in such a way
that they do not modify or depend upon any other word in the sentence.
When we analyze our sentences, these independent words are not
considered as elements of the sentences in which they are used. It is
sufficient to say that they are independent words.
4. +Conjunctions used as introductory words.+ We have noted the use of
conjunctions like the co-ordinates _and_, _but_, etc., and the
subordinates _because_, _in order that_, _so_, _for_, _wherefore_,
_how_, _whether_, etc., which are used to introduce sentences and
connect them in thought with sentences and paragraphs which have gone
before.