That's how it is, I used to…: 〜ものだ

〜ものだ has three uses. It states a general truth or a rule of conduct: 人の話はよく聞くものだ, “you should listen to others” (in the negative: 〜ものではない, “that's not done”). With a verb in the past, it calls up a memory with nostalgia: よく川で遊んだものだ, “I used to play in the river so often”. Finally, it expresses feeling about the way something is: 時間がたつのは早いものだ, “how time flies”.

• 人の話はよく聞くものだ。ひとのはなしはよくきくものだ。You should listen carefully to what others say (it's the rule).
• 子どものころ、よくこの川で遊んだものだ。こどものころ、よくこのかわであそんだものだ。As a child, I used to play in this river so often.
• 時間がたつのは早いものだ。じかんがたつのははやいものだ。How time flies!

もの means “thing”, and by extension “the nature of things”. 〜ものだ therefore says: “it's in the nature of things”. Its three uses grow out of that: the rule (that's how it's done), the heartfelt observation (that's how life goes) and the memory (that's how it used to be).

• 学生のころは、よく友だちと朝まで話したものだ。がくせいのころは、よくともだちとあさまではなしたものだ。As a student, I'd often talk with my friends until morning.

The verb form and the context tell you the use: present for a rule or an observation, past (with よく) for a memory — like English “used to” or “would”.

• 年上の人には、丁寧に話すものだ。としうえのひとには、ていねいにはなすものだ。You speak politely to your elders (that's the rule).
• 人の悪口を言うものではない。ひとのわるくちをいうものではない。You don't speak ill of others; it's just not done.

In the present, with an adjective, ものだ expresses feeling about a truth you sense: “how… it is!”. You often hear it from older people. In speech, ものだ becomes もんだ.

• 京都の秋は、何度見てもいいものだ。きょうとのあきは、なんどみてもいいものだ。Autumn in Kyoto — you never tire of it, however many times you see it.
• 子どもはすぐ大きくなるもんだね。こどもはすぐおおきくなるもんだね。Children grow up so fast, don't they.

ものだ states a general rule, valid for everyone (“you should”). ことだ (line 10) gives personal advice suited to someone's situation (“the best thing for you is to”).