Evaluation and Judgment Japanese Grammar Lessons

普通形＋と思う lets you report your own thought, belief, or judgment by putting a plain-form clause before と思う. The embedded clause stays plain even when the whole sentence is polite with と思います; politeness lives on the outer verb, not inside the thought. At N4 you use this pattern constantly for opinions, soft claims, and everyday expectations such as weather, plans, and impressions.

～ものだ looks at a situation as a general truth, a soft “one should,” or a nostalgic past habit, depending on the form and the context. 人は誰でも失敗するものだ means people fail—that is just how it is. 約束は守るものだ softly says one should keep a promise. 遊んだものだ looks back fondly on a past habit. Attach ものだ to a plain form: verb, i-adjective, or na-adjective + な. Polite speech uses ものです, and casual speech often contracts to もんだ.

～ものではない says that an action goes against common sense (“one should not…”) or that something is, by its nature, not like that. 人の悪口を言うものではない is a common-sense warning: (one) should not speak ill of others. 外国語は一か月で身につくものではない says that a foreign language is not something (one) masters in a month. Attach ものではない to a plain form: a dictionary-form verb, an i-adjective, or a na-adjective + な. Polite speech uses ものではありません, and casual speech often uses もんじゃない.