請問這句is love i can be sure of是關代還是從屬句,如果是關代那就是省了that在love後.但它好像是問句,如果是從屬句,那that應該不能省的才對?謝謝!?

He is sure of his success.
He is sure (of) that he will succeed.因為that子句（附屬子句）不受介系詞的約束，所以sure後面不可使用介系詞，故of遭強制刪去。that是帶出名詞子句的「連接詞」。

Is love (that) I can be sure of?
這是一個complex sentence，主詞補語是一個名詞子句。因為問句結構而倒裝（be動詞移主詞love之前）。
這裡的that是「指示代名詞」，屬於名詞子句裡介系詞of的「受詞」。
當作附屬子句受詞的that可以省略或保留。

Love is what you can be sure of.主詞＋be動詞＋主詞補語（名詞子句）
what是「帶先行詞的關係代名詞」
what = the thing that
Love is that thing that you can be sure of.主詞＋be動詞＋主詞補語＋修飾語（修飾前面的名詞）

In this pattern the object of the verb is a (dependent clause) which is introduced by a relative adverb or pronoun , whether  or  "if"
I can be sure of------of forming adj phrase descriptive----main cl.---S + vt
"if" it is love-------(dependent clause)--no need "that"/may be a question.pattern also on rearrangement..