(a) These are the lines which heaven-commanded Toil shows on his deed.
(b) And since that time I thought it not amiss To judge which were the best of all these three.
In sentences 1 (a), 2 (a) and 3 (a) the regular relative use is seen; who having the antecedent gentleman, what having the double use of pronoun and antecedent, which having the antecedent lines.
But in 1 (b), 2 (b), and 3 (b), there are two points of difference from the others considered: first, no antecedent is expressed, which would indicate that they are not relatives; second, a question is disguised in each sentence, although each sentence as a whole is declarative in form. Thus, 1 (b), if expanded, would be, "Who stood behind? We knew," etc., showing that who is plainly interrogative. So in 2 (b), what is interrogative, the full expression being, "But what had become of them? They knew not." Likewise with which in 3 (b).
How to decide.
In studying such sentences, (1) see whether there is an antecedent of who or which, and whether what = that + which (if so, it is a simple relative; if not, it is either an indefinite relative or an interrogative pronoun); (2) see if the pronoun introduces an indirect question (if it does, it is an interrogative; if not, it is an indefinite relative).
Another caution.
128. On the other hand, care must be taken to see whether the pronoun is the word that really asks the question in an interrogative sentence. Examine the following:—
1.
Sweet rose! whence is this hue
Which doth all hues excel?
—Drummond