The thing about “thing” is that it’s a difficult thing to define. There are physical things, metaphorical things, things to do, things to consider, thingamabobs and so on. Unfortunately, things don’t get any less complex in Japanese.
While the kanji versions of 物 (mono) and 事 (koto) more commonly refer to “things” in the strictest syntactic sense, there are plenty of ways to use もの and こと in hiragana for other purposes that don’t quite translate as directly. For example, it’s no rare occurrence to hear ものだ or ことだ at the end of a sentence — but these phrases hardly have anything to do with “things,” tangible or otherwise. Rather, ものだ and ことだ are used to admonish, give advice, or simply state an opinion on life.
As the more objective of the two phrases, ものだ is used to express how society typically views the world and its machinations. For example, we all know to mind our manners in delicate environments, hence we would say, “病院では静かにするものだ” (“Byōin de wa shizukani suru mono da,” “Best be quiet in the hospital”).
Likewise, many among us propound that nobody can predict what tomorrow may bring and so they say, “未来のことは誰にもわからないものだ” (“Mirai no koto wa dare ni mo wakaranai mono da,” “Nobody knows what the future holds”).
Conveniently, ものだ is grammatically diverse. It can be added to the dictionary form or negative conjugation of a verb, as in the above examples, or it can connect to i-adjectives and na-adjectives such as with 豪雨のあとの森はやはり静かなものだ (Gōu no ato no mori wa yahari shizukana mono da, Forests are all the more quiet after a rainstorm) and 政治は所詮騒がしいものだ (Seiji wa shosen sawagashii mono da, Politics are a noisy affair after all).
On the other side is ことだ. Its meaning is closer to ～べき (~beki, should) or ～なければならない (~nakereba naranai, must) in that it allows the speaker to provide advice or admonishment on a particular topic, albeit slightly less directly than via a straight command.
ことだ is primarily used in direct response to a situation brought about by the other party in the conversation. For example, seeing a young colleague arrive at the office soaking wet might spur a stern boss to say “天気予報を確認してから出かけることだね” (“Tenki yohō o kakunin shite kara dekakeru koto da ne,” “You should always check the weather forecast before leaving the house”). In another instance, “出来上がったばかりの串焼きをすぐに食べないことだね” (“Dekiagatta bakari no kushiyaki o sugu ni tabenai koto da ne,” “Everyone knows not to eat the skewers straight off the grill”) would be a comical albeit fitting lecture to give to a friend who is now frantically fanning at the burning meat in their mouth.
The grammatical nature of ことだ allows it to connect only with verbs, either in dictionary form or their negative conjugation. This makes using ことだ somewhat more restrictive, although admonishment typically centers around what someone should or shouldn’t do, so the linguistic similarity to English is rather helpful.
Naturally, there is much more complex grammar awaiting us in the world of Japanese, but for now, 言語をマスターするには簡単なものから始めることだ (Gengo o masutā suru ni wa kantanna mono kara hajimeru koto da, Mastering a language means starting with the simple things).