This paper discusses the use of hazu da in essays stating one's opinion written by learners of Japanese, in comparison with those by native speakers. The analysis of the composition structure of essays by native speakers shows that hazu da tends to be used in three types of discourse pattern. In each type hazu da demonstrates a specific function required by specific logical development as shown below.

　(1) Type α: ＜present situation＞⇒ suggestion {if you take it, then} hazu da sentence

　　The function of hazu da : to support the author's suggestion by saying that a desirable situation will be highly feasible if it is accepted.

　(2) Type β: ＜present situation＞⇒ hazu da sentence {therefore / nevertheless} suggestion

　　The function of hazu da : to support the author's suggestion by showing the appropriateness of what the sentence with hazu da says.

　(3)Type γ: hazu da sentence {but} resent situation (contradicting hazu da sentence)

　　⇒＜opinion / consciousness raising of a problem＞

The function of hazu da: to maintain critically that the present situation is not a proper one if compared with what otherwise should be.

The analysis of the composition of essays by less advanced learners of Japanese, however, reveals that many of them could not properly use the right types of hazu da in a specific context. From these findings, this paper insists that it is vitally important for teachers of Japanese as a FL/SL to provide learners with explanation of the functions of hazu da and their selection patterns in a specific context in order to enable them to write convincing essays stating their opinions.