訳(わけ)だ

One of the most misunderstood words in Japanese is 訳(わけ). Of the many ways it can be translated, the most common are 'reason', 'cause', 'meaning', or 'conclusion'. Despite this, it may be beneficial to consider it almost identical to an equally versatile English word 'so'.
In all of its uses, わけ simply highlights the plain existence/determination of something in the same way that 'so' does in English. This can be seen in its most basic form in わけだ. A combination of わけ and the auxiliary verb だ.
わけだ will be used at the end of sentences, and is sometimes preceded by という. This use is often translated as 'for the reason of (A)', 'no wonder its (A)', or 'its (A), as you'd expect'. Realistically though, it just means 'it is so that it's (A)', or 'so it's (A)'.

• 事故(じこ)で電車(でんしゃ)が遅(おく)れたの？だから珍(めずら)しく遅(おく)れて来(き)たというわけだ。
  The train was late due to an accident? So that's why you came in unusually late.
• 指(ゆび)の骨(ほね)が折(お)れていたのですね。だから指(ゆび)を動(うご)かしたら痛(いた)かったという訳(わけ)ですね。
  You are saying that my finger was broken? So that's why my finger hurt when I moved it.
• やっぱり全然(ぜんぜん)練習(れんしゅう)してないでしょ？どうりで一年(いちねん)たっても下手(へた)なわけだ。
  As I expected, you didn't practice at all, did you? No wonder you haven't improved even after a year.
• 今日(きょう)は休日(きゅうじつ)か。だから定休日(ていきゅうび)のわけだ。
  Today is a holiday, isn't it? So that's why they are closed today.

Due to わけ being a noun, it will need to follow the attributive form of whatever word comes before it. As we can see 'so' and わけ have an extremely close pattern of nuances, ranging from 'because', all the way through to simple emphasis (like with 'naturally', and 'as you'd expect').
わけ may also be used in questions, implying that the speaker is trying to determine something that they have heard. わけだね 'so its (A) right?', and わけですか 'so is it that (A)?' are good examples of this.

• 俺(おれ)と一緒(いっしょ)にいたくないわけだね?分(わ)かったよ。
  So you are saying that you don't want to be with me, right? I understand.
• 先輩(せんぱい)たちは後(あと)から来(く)るというわけですか？
  Senpai, so will you be coming later?