Could someone please explain to me the different meanings that ものだ can have at the end of a sentence ? Here are some examples I understand the general meaning of, but I don't understand the logic/grammar behind ものだ
- 留学したいものだ
- 約束は守るものだ
- 父は以前はバスで仕事に行ったものだ
- 生徒はもっと勉強するものだ
There are four basic usages/meanings of the sentence-ending ものだ/ものです/ものである, etc. In colloquial speech, 「もの」 often changes to 「もん」.
1) Exclamation, deep emotion, surprise, praise, etc.
2) General tendencies and cold facts.
3) Advice based on common sense, customs, etc.
4) Recollection and reminiscence.
The context in which 「～～ものだ」 appears should tell you which usage/meaning it is being used for. My advice would be to not over-analyze the short phrase 「ものだ」, which consists only of two words. Instead, try to get used to the usages by reading a lot and forming many example sentences yourself.
Now, let us take a look at the sentences you have listed.
「留学{りゅうがく}したいものだ。」 "I really would like to study abroad."
would clearly be type #1 above. The speaker's wish to go study abroad is rather strong here.
「約束{やくそく}は守{まも}るものだ。」 "People should keep their promises."
would be type #3 because that is common sense.
「父{ちち}は以前{いぜん}はバスで仕事{しごと}に行{い}ったものだ。」 "Daddy used to commute to work by bus."
Type #4, no sweat. It is about how something used to be done. The key phrase is 「以前は」 with the contrastive 「は」. The father commuted by bus before, but not anymore.
「生徒{せいと}はもっと勉強{べんきょう}するものだ。」 "Students should study harder."
This would be a split between types #2 and #3. A larger context would probably tell us which one.
The second and fourth ones are the case where it is kind of a general rule or expectation. It translates as "should (always)", and could be replaced with 〜べき.
- 約束は守るものだ → You should always keep your promises.
- 生徒はもっと勉強するものだ → (The) Students should study harder.
