In short

〜でしょう (deshō) is “probably” - your reasoned guess; 〜かもしれない (kamoshirenai) is “maybe, it’s possible” - a weaker one. Both attach to the plain form, and both leave nouns and な-adjectives bare: 雨でしょう (ame deshō, it’ll probably rain), 静かかもしれません (shizuka kamoshiremasen, it might be quiet) - never a だ in between. Said with a rising intonation, 〜でしょう？ stops being a guess and asks the listener to confirm.

The rule

Step 1 - Attach to the plain form.

The base is the plain form from lesson 17. Politeness is carried once, on the outside - でしょう and かもしれません are already polite, so the verb inside them stays plain even though the sentence ends politely.

• 降る furu -> 降るでしょう furu deshō (it will probably rain)
• 降らない furanai -> 降らないかもしれません furanai kamoshiremasen (it might not rain)
• 高い takai -> 高いでしょう takai deshō (it’s probably expensive)

Step 2 - Nouns and な-adjectives attach bare - no だ.

This is the one form error worth drilling. でしょう is itself the tentative form of です, so the copula is already inside it; a second だ has nothing to do:

• 雨だ -> 雨でしょう ame deshō (not ×雨だでしょう)
• 静かだ -> 静かでしょう shizuka deshō (not ×静かだでしょう)

かもしれない follows the same bare rule (学生かもしれません gakusei kamoshiremasen): what goes in front of it is the plain predicate without だ, so a noun or a な-adjective stem attaches directly. Past and negative forms already end like a plain predicate, so they attach directly: 雨だったでしょう, 雨じゃないでしょう.

Step 3 - でしょう or だろう: same grammar, different coat.

でしょう is the polite shape and your default. だろう (darō) is the plain shape - casual speech and plain-style writing. In conversation だろう leans masculine and blunt; in essays, novels and reports it is simply the neutral written form. Contracted casual speech shortens both: でしょ (desho) and だろ (daro).

Step 4 - Falling = probability, rising = confirmation.

The same three syllables do two jobs, and only the pitch separates them:

• 明日は雨でしょう。 falling - It will probably rain tomorrow. A guess about the world.
• 明日は雨でしょう？ rising - It’s going to rain tomorrow, right? A push for agreement.

The rising one overlaps with ね from lesson 11, but they lean differently. ね invites you to share a feeling about something you can both see (いい天気ですね ii tenki desu ne). でしょう？ presses for confirmation of something you believe and think the listener can verify - which is why it also carries a hint of “I told you so”: ほら、言ったでしょう？ (Hora, itta deshō?) See, I told you, didn’t I?

Step 5 - The adverbs that announce a guess.

Japanese is verb-final, so the modality arrives last. An adverb at the front warns the listener early, and the ending is then expected to match it.

• たぶん tabun (probably) - the everyday partner of でしょう.
• おそらく osoraku (probably) - the same meaning in formal or written register.
• きっと kitto (surely, I’m sure) - stronger; a confident でしょう, often with an emotional charge.
• もしかしたら／ひょっとしたら moshikashitara / hyottoshitara (possibly, it just might be) - the partners of かもしれない.

So: たぶん〜でしょう, きっと〜でしょう, もしかしたら〜かもしれません. Pair them the other way round - もしかしたら〜でしょう, きっと〜かもしれません - and the sentence contradicts itself, promising one degree of confidence and delivering another.

Note the asymmetry. もしかしたら and ひょっとしたら genuinely need a possibility ending to land on; without かもしれない they sound unfinished. たぶん, おそらく and きっと only lean that way - they can also stand in front of a flat statement (たぶん今日は帰りが遅くなります Tabun kyō wa kaeri ga osoku narimasu, I’ll probably be back late today; きっと勝ちます Kitto kachimasu, I’m sure we’ll win). 〜でしょう is the safe default, not a grammatical obligation.

Step 6 - でしょう is the register of forecasts and news.

Weather reports and news read almost entirely in でしょう: 午後から晴れるでしょう (Gogo kara hareru deshō), 明日は寒くなるでしょう (Ashita wa samuku naru deshō). A broadcaster must not assert what has not happened, so でしょう is the professional default, not hedging. Listen to any Japanese forecast and you will hear it a dozen times.

Step 7 - かもしれない and casual かも.

〜かもしれない / polite 〜かもしれません means “might, may, it’s possible” - a real but uncommitted possibility. It is one fixed unit: you cannot inflect しれる freely inside it, and the polite form replaces ない with ません. In casual speech it is clipped to かも (kamo), which is extremely common as a soft hedge, even as a one-word answer: 「行く？」「かも。」 (“Iku?” “Kamo.”) “Coming?” “Maybe.”

Step 8 - のではないでしょうか: an opinion in soft clothing.

Literally “is it not the case that…?”, 〜のではないでしょうか (no de wa nai deshō ka) is how you offer an opinion politely in meetings and writing - it states your view as a question so nobody has to lose face. It builds on 〜んです from lesson 20. More colloquial speech contracts の to ん: 〜んじゃないですか (still polite) and, dropping the ですか, casual 〜んじゃない？.

What this lesson is not. 〜そうだ, 〜ようだ and 〜らしい also translate as “seems / apparently”, but they are evidence-based: they report what you saw, heard or were told. でしょう and かもしれない report your own reasoning, with no evidence claimed. Those are lesson 42 and lesson 49. The stronger end of the scale - はずだ and に違いない - is lesson 53.

Forms & patterns

Attachment - the plain form goes in, nouns and な-adjectives lose their だ:

Politeness ladder:

The certainty scale - pick by how much you are willing to be wrong:

Matching adverb to ending:

Examples

Common mistakes

• 明日は雨だでしょう。 (Ashita wa ame da deshō.) -> 明日は雨でしょう。 (Ashita wa ame deshō.) - でしょう is the tentative です, so the copula is already inside it. Nouns attach bare.
• 静かだかもしれません。 (Shizuka da kamoshiremasen.) -> 静かかもしれません。 (Shizuka kamoshiremasen.) - な-adjectives drop だ here too. The か of かも cannot follow だ.
• 雨が降りますでしょう。 (Ame ga furimasu deshō.) -> 雨が降るでしょう。 (Ame ga furu deshō.) - でしょう follows the plain form; the politeness is carried once, by でしょう itself.
• 遅れるかもしれます。 (Okureru kamoshiremasu.) -> 遅れるかもしれません。 (Okureru kamoshiremasen.) - かもしれない is a frozen unit. Its polite form swaps ない for ません; there is no ×かもしれます.
• もしかしたら彼は来るでしょう。 (Moshikashitara kare wa kuru deshō.) -> もしかしたら彼は来るかもしれません。 (Moshikashitara kare wa kuru kamoshiremasen.) - もしかしたら opens a bare possibility, so it has to land on かもしれない. でしょう asserts a probability and clashes with the adverb that set the sentence up.

Tips

• でしょう is not the past of です. でした deshita (was) and でしょう deshō (probably is) look almost identical and mean completely different things. 学生でした = he was a student; 学生でしょう = he’s probably a student.
• English puts “maybe” first, Japanese puts the guessing last. Because the modality lands on the final word, a Japanese sentence stays neutral until the very end - which is exactly why たぶん and もしかしたら exist at the front. Say the adverb, then commit to the matching ending.
• でしょう？ is a question about the listener’s knowledge, ね is about shared feeling. Use ね when you both perceive the same thing (暑いですね); use でしょう？ when you are betting on something the listener can confirm and you cannot. To a superior, ね is the safer of the two - でしょう？ can sound like you are cornering them.
• かも is the everyday hedge. In casual conversation 〜かも does the work that “I dunno, maybe” does in English, and it softens refusals: ちょっと難しいかも (Chotto muzukashii kamo) is a gentle that might be tricky - which usually means no.