Let me explain simply. If you search for them on the Internet, you will find detailed answers. - 今後 and これから (don't they both mean "from now on"?) The former sounds more formal. https://hinative.com/ja/questions/4054624 - 恐らく and 多分 (don't they both mean "perhaps" or "probably"?) The former sounds more formal. https://hinative.com/ja/questions/4054624 - 友人 and 友達 (don't they both mean "friend"?) The former sounds more formal. https://hinative.com/en-US/questions/4897 - 小さな and 小さい (don't they both mean "small"?) I am not quite sure (Sorry). When a noun comes after that, they are interchangeable (e.g., 小さな本，小さい本). However, you can't finish a sentence with 小さな while you can with 小さい. (この本は小さい) See; https://hinative.com/en-US/questions/3907 - 使う and 用いる (don't they both mean "to use"?) The latter sounds more formal. - 上手い and 上手 (don't they both mean "skillful"?) They sound the same to me. Just the pronunciations are different. https://hinative.com/ja/questions/171920 - 理解する and 分かる (don't they both mean "to understand"?) The former sounds more formal but I feel they are a little different. See https://www.italki.com/question/156645 - 中 and 内 (don't they both mean "inside"?) 中; you can't see the inside. 内; you can see the inside. http://lang-8.com/568038/journals/117933294473342533157826931788745017376

*** 「内（うち）」ｖｓ「中（なか）」 - 中：location (inside) - 内：mental location (not real one) - 内側：location (inside) ------------------------------------------------------------- *** 「うち」とは？ What is 'UCHI' ？ 「自分を中心として心理的な円を描いたときの自分側」を「ウチ」といい、向こう側を「外（ソト）」といいます。 when you draw a mental circle about you, the area inside is called 'ウチ', the outside 'ソト' -　「内」が現実の場所を指すために使われることはほぼありません。現実の場所を指す場合は、「内側」を使います。 Most of the usage of '内' does not indicate real location. if you want to refer to the location, usually you use「内側」. 例：「コースの内側を走る」「太ももの内側にTatooを入れる」「このドアは内側から開くタイプです」 ------------------------------------------------------------- *** 例文1: examples -　心の内：(deep) in my heart -　心の中：in my heart -　胸の内：(deep) in my heart -　胸の中: physically inside the breast - ねえ、ウチ来る？：don't you want to come to my house? - ウチの会社：our company - ウチのにゃんこ：My kitty, Kitty in my house - ウチ：I (Kansai dialect, a girl/woman call herself 'ウチ') 　 ウチ、もうめっちゃ感動！（I'm really moved!) - おうち（お家）：house - 身内(みうち）： family or people like a family - ウチら：we - 内輪(うちわ）:「内輪のことまであなたに話したくない」 　　　　i don't want to talk to you about our familly thing. - 内内で：内内で（結婚）式をすます。 have a wedding feast only with family - 内幕：behind the curtain. 「内幕を暴露する」 - ウチのダンナ（my husband) *「内側のダンナ」*「中のダンナ」 cannot be said. ------------------------------------------------------------- *** 例文2: - 城壁(castle wall)の内に閉じ込められた。 　 The enemy drived you inside the castle wall and shut the gate. - 城壁の中に閉じ込められた（塗り込められた）。 　　The enemy put you in the wall, and paint the plaster on you, and you'll be buried. (So horrible) ※ castle wall has a circle form. ------------------------------------------------------------- *** 例文3: - この中／内側　inside of this - その中／内側　inside of it - あの中／内側　inside of that - このうち　among these 「A,B,C,D. このうち正解はどれでしょう？」 - そのうち　someday 　「そのうち何とかなるよ」 - あのうち　(※ no such a word) 　※ 「うち」cannot be used for things/situation far away from you. -------------------------------------------------------------

Let me introduce "our" hypothesis to your questions in Japanese. The reason why I said "our" is, while I was tackling your question, one of my co-workers who has majored in Japanese liguistics kindly helped me a lot. You actually have a lot of supporters to rely on! - 恐らく and 多分 (don't they both mean "perhaps" or "probably"?) As you mentioned, both refer to probability. However, when you look them up in the Japanese-Japanese dictionary, it says; The word 恐らく seems that a speaker has kind of doubt that sth won't happen, but you anyway assume that it could happen. The word 多分 seems that a speaker believes that sth will happen with higher probability than 恐らく. So my kind co-worker suggested that it depends on how much the speaker believes it will happen. - 小さな and 小さい (don't they both mean "small"?) Both mean small, however, 小さな sounds more subjective. We use 小さな with a feeling which is related to "small" cute, immature, or timid etc... 小さい sounds more objective. It does literally mean "small". So, for example, when you write a literary essay like a thesis, 小さい is preferable. On the contrary, if you look at emotional materials like lyrics of Japanese, you probably will find more 小さな in them. - 使う and 用いる (don't they both mean "to use"?) Yes, both mean "to use". You can use 使う more broadly. If you don't know which to use, just use 使う for safe. On the other hand, 用いる especially means " to use sth as a useful thing recognizing its value. - 中 and 内 (don't they both mean "inside"?) Both mean inside, but the difference between them would be its strictness, we would say. 中 means inside, or even between, among. So, to use 中, it should be loosely enclosed. Compared to 中, 内 means the inside which is more enclosed. For other pairs in your questions, I totally agree with other helpers. Have a good Japanese saturated day!

One additional comment: -　理解できる：他動詞 　　　　→　日本語の文法【を】理解する。 　　　　　　　　　NG:　*(文法が理解する) -　分かる：自動詞 　　　　→　日本語の文法【が】分かる 　　　　　　　　　NG:　*(文法を分かる) ※　分ける（他動詞）： divide something, share, pigeonhole (みんなでお菓子を分ける, 細かく分ける) 分かる（自動詞）： divide itself -> understand (become understandable) (by the way, "わかれる（別れる）" means "get separated, say good bye each other", and the root of the word is same as 'わかる（分かる）' （Aが）分かる: it's like 'A big mass of A is dividing itself (automatically) into small pieces , which you can easily deal with' -> turned into the meaning of 'understand/understandable' The point is: The subject of the sentence Aが分かる is not you, but A. (A divides itself into pieces in front of me -> I understand A) while that of （私は）Aを理解する　is "I". (I understand/catch/analyse A) ※ FYI: The kanji '解' orinigally means 'dissection of animal'.

Wow, what an incredible number of replies! You all have been very kind, thank you so much!!!