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バスに乗りましょう。タクシーは高いですから。 

or can you simply say タクシーは高いから without the です?

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1  
You can also compare this to タクシーは高いからです. –  Earthliŋ Jun 11 at 11:37
    
I would not expect to hear ◯◯ は高いですから when making a statement of that kind. I would expect it more as a response to a question or if you plan to say "because the taxi is expensive, lets take the bus", while if you are saying "lets take the bus because the taxi is expensive", id go with what was posted above ◯◯ は高いからです. edited to punctuate better. –  VampyreSix Jun 12 at 17:45
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@VampyreSix 「バスに乗りましょう。タクシーは高いですから。」 sounds perfectly natural. It's the polite version of 「バスに乗ろう。タクシーは高いから。」. 「バスに乗りましょう。タクシーは高いからです。」 doesn't sound natural. It's like saying 「バスに乗ろう。タクシーは高いからだ。」 –  Choko Jun 12 at 22:56

3 Answers 3

up vote 5 down vote accepted

As Tomiyoshi notes, です is a polite marker, much like the -ます ending on verbs. The plain form is だ, corresponding to the dictionary or plain form of verbs. In technical terms, this です・だ word is often called a "copula" (after -i adjectives, です is instead regarded as a politeness marker). Whether either form is required at the end of a sentence depends on two things: 1) the grammatical structure, and 2) the social register (i.e. the context, who you're talking to).

Grammar

Generally speaking, if a sentence ends in a noun or a -na adjective, you need to use the copula (either だ or です, depending on social register).

If a sentence ends in an -i adjective, as in your sample sentence, adding だ in informal contexts is actually considered incorrect grammar, since the -i adjective is already grammatically complete on its own. In technical terms, an -i adjective forms a complete predicate (i.e. it works as a verb), so the copula is not needed. However, ‑i adjectives do not conjugate for politeness (there is no -ます form), so in polite contexts, です is used as a politeness marker, as a kind of replacement for -ます.

Social register

If you're among friends, だ after nouns or -na adjectives might be appropriate as the less-formal plain form. If you're among strangers, colleagues, or in a professional setting, you should use です, also after ‑i adjectives.

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Following an -i adjective, I think that in standard Japanese です has been reanalyzed as a politeness marker rather than a copula. It doesn't bear tense, the adjective does: 高かったです but not *高いでした; it's not in alternation with the plain form copula: 高いです or 高かったです but not *高いだ or *高かっただ; and 高い forms a complete predicate on its own. So in this case I would not call it a copula or a form of だ. –  snailboat Jun 11 at 6:05
    
@snailboat: For purposes of this answer, is it acceptable to call it a "copula" for sake of simplicity, since that's what です is in other contexts? Or do you think the distinction is important enough to merit editing? –  Eiríkr Útlendi Jun 11 at 6:12
    
I tend to agree with snailboat. I think the distinction is important to make, because if です were a copula, dropping it would be grammatically incorrect. –  Ataraxia Jun 11 at 12:38
    
@Ataraxia, snailboat, let me know if this edited version works. I tried to clarify without getting bogged down in the minutiae. –  Eiríkr Útlendi Jun 12 at 19:06

Yes, it's grammatically correct. です is just a polite expression. I would prefer to add です when I am in a formal situation or with senior people.

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Do you need です before から at the end of sentence?

Not necessarily. It depends on how polite you want to be.

バスに乗りましょう。タクシーは高いですから。
or can you simply say タクシーは高いから without the です?

Yes, both are correct. You can say:

バスに乗りましょう。タクシーは高いですから。(polite form)
バスに乗ろう。タクシーは高いから。(casual form)

because these are inverted sentences of:

タクシーは高いですから、バスに乗りましょう。(polite form)
タクシーは高いから、バスに乗ろう。(casual form)
(タクシーは高いから、バスに乗りましょう。is also fine.)


(You don't say *バスに乗りましょう。タクシーは高いからです。/*バスに乗ろう。タクシーは高いからだ。 because you don't say *タクシーは高いからです、バスに乗りましょう。/*タクシーは高いからだ、バスに乗ろう。You'd use タクシーは高いからだ。/タクシーは高いからです。 when you say "It is because taxi is expensive", as a response to "Why do you take a bus?" etc.)

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