Por que falam "I do" ao invés de "I Think"?

O diálogo que vi era exatamente assim:

- You think so ?
- I do.

Por que ele respondeu “I do” ao invés de falar “I think” no sentido de “eu acho” ?

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Nesta aula, o professor Denilso de Lima, autor do livro "Combinando Palavras em Inglês", ensina como as collocations (combinações de palavras) podem ajudar você a falar inglês com mais naturalidade. ACESSAR AULA
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NeyF 3 24 206
Essa é uma característica do inglês. Respondemos com "I do" ou "I don't", quando a resposta é sim ou não.

Do you like it? (Você gosta disso?)
Yes, I do. (Gosto)
No, I don't. (Gosto não)

Do you think so? (Você acha que sim?)
Yes, I do. (Acho)
No, I don't. (Acho não)

Do you have a pencil? (Você tem um lápis?)
Yes, I do. (Tenho)
No, I don't. (Tenho não)
PPAULO 6 49 1.3k
This reminds me of the ultimate question:
Do you Jane Doe, take this man Johe Doe, to be your lawful wedded husband. To have and to hold, to love and to cherish, for richer or poorer, in sickness and in health, from this day forward. 'til death do you apart?

Do you John Doe, take this woman Jane Doe, to be your lawful wedded wife. To have and to hold, to love and to cherish, for richer or poorer, in sickness and in health, from this day forward. 'til death do you apart?
To which, both certainly will say/answer "I do".
So "I do" would mean that they will "take" their significant other and do what is asked to. This way , "do" would replace "take" (in "take this man [or woman] to be your lawful wedded husband/wife.").

As NeyF pointed out, a question you could answer with Yes or No.

Granted, with the 3 versions of the Anglican Church the officiant asks as he asked to the Prince William:
"William Arthur Philip Louis, wilt thou have this woman to thy wedded wife, to live together according to God's law in the holy estate of matrimony? Wilt thou love her, comfort her, honor and keep her, in sickness and in health, and forsaking all other, keep thee only unto her, so long as ye both shall live?"
To what the groom respond "I will".
Then similar question comes to the bride, to what she will answer "I will".