I wrote the correct translation of them. But I repeat again. This sentences that I write here is our grammatically words. we don't use them in our conversations. You just see them at Persian books.
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3. It is bad.
این بد است. ( It in persian be "این" and even you can remove it )
4. These clothes are clean.
این لباسها تمیز هستند. ( it's okay to use single verb when you talk about things but It's better to use plural verb. )
5. That old woman is still beautiful.
آن پیرزن هنوز زیبا هست ( Old Woman = پیرزن , Old Man = پیرمرد )
6. A dirty dress is very bad.
لباسِ کثیف خیلی بد است ( Dirty = کثیف )
9. How is it?
چطور است ( Remove "it" from the sentence and translate it )
11. Which student is here?
کدام شاگرد اینجاست
12. Who is ugly?
کی زشت هست ( Who is ugly = کی زشت هست؟ , Who is the ugly one = زشت کیست؟ )
18. The old shoes are here.
کفشهای کهنه اینجاست
19. Where is the wise old man?
پیرمرد دانا کجاست؟
( please read grammar of adjective in persian. you can't write " مرده بد ". because it means " Bad dead body ". You should write " مردِ بد " and it means " Bad man ".)
20. You are still a little too fat.
تو هنوز یک ذره چاق هستی. ( little = کوچک , a little = کمی ، یک ذره )