What’s the difference between “tabun” (たぶん) and “desho” (でしょう)?

Question

What’s the difference between “tabun” (たぶん) and “desho” (でしょう). Both mean “probably”, but when would you use one over the other? Please explain by referring to the following examples:

Example 1

たぶん、黒とグレーです。
Tabun, kuro to guree desu. (It is probably black and gray.)

Example 2:

私たちがうわさをしているのではと彼女が心配するでしょう。
Watashi tachi ga uwasa o shite iru no dewa to kanojo ga shinpai suru deshou. (She will probably worry if we are gossiping about her)

Please note: I am a beginner and so would appreciate a simplified explanation. Thanks!

Answer 1

I am a native speaker of Japanese but not of English. So my English may not be natural but I hope to be helpful.

たぶん and でしょう have similar meanings but are different parts of speech.

たぶん（=おそらく）'s definition is:

たぶん
1 〔おそらく〕probably; perhaps; possibly; maybe

So たぶん is an adverb like probably, perhaps, or possibly. And たぶん’s meaning varies from probably to possibly according to contexts.

でしょう（=だろう）’s definition is:

だろう
1 〔推量〕
(1)〔話者を主語にして〕I think [suppose/guess] (that).... [...]
(3)〔推量の助動詞を用いて〕

推量の助動詞 means can be, may be, could be, will be, etc.

So でしょう is an (auxiliary) verb like think, suppose, guess, will be, or may be.

Therefore you can say:

明日はたぶん雨だ。 (It probably rains tomorrow.)
明日は雨でしょう。 (It will rain tomorrow.)
明日はたぶん雨でしょう。 (It will probably rain tomorrow. Or I think it probably rains tomorrow.)

(I quoted the definitions from プログレッシブ和英中辞典)

Answer 2

たぶん and でしょう belong to different word classes, and it's hard to directly compare them. たぶん is an adverb, and でしょう is a "form of です" that inherently has a speculative/inferential meaning. Asking about the difference between them is like asking about "probably" and "I suppose". Actually, you can safely use both たぶん and でしょう in one sentence:

たぶん彼は心配するでしょう。
He will probably worry. / I suppose he will worry.

The English word "probably" is an adverb, so you can say たぶん is closer to "probably". You can think でしょう is a way to express a similar idea without using an adverb. Finally, でしょう has various functions, so you may want to review the use of でしょう in a longer article like this.