Por que não se usa o auxiliar ''do'' neste caso?

For a backache, it's important to take some pain medicine.
For a sunburn, it's sometimes helpful to put some cold tea on it.

Why don't we use the auxiliary verb "do" in the examples above?

I learned English abroad and I didn't actually go through the grammar rules thoroughly. I know we can't place "do" in these sentences and that they would sound really odd, however, I've been helping out some friends with English and I couldn't tell them the exact reason why.

Thanks,

Vini
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Olá Vinicius.

O auxiliar do é utilizado em frases interrogativas e negativas. Para frases afirmativas não há necessidade de utilizá-lo. :)
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Hi Vinicius!
(complementando)
I'll try to explain it to help a little bit.

The auxiliary do/does usually is written in questions and negative sentences , we use the auxiliary in affirmative sentences to emphasize,

Look:

Do you take medicines? (question)
No, I don't take medicines. (negative)
Yes, I take medicines. (affirmative)

Yes, I do take medicines. (emphasizing)

However, the sentences you wrote are:
For a backache, it's important to take some pain medicine.
For a sunburn, it's sometimes helpful to put some cold tea on it.


Both sentences contain "to be", below I wrote some questions in which "to be" is used as auxiliary:

Is this important to take some pain medicines?
Why is it important to take some pain medicines?

Is it helpful to put some cold tea on it?
When is it helpful to put some cold tea on it?

You can see that I used to be to make sentences using the context you presented.
In the sentences I wrote above I couldn't use "do/does"

Cheers!