The verb “to be” is بودَن (būdan), which makes the past tense بود (būd) and the simple past conjugation:
- “I was” = بودَم (būdam)
- “you (sing.) were” = بودی (būdī)
- “he/she/it was” = بود (būd)
- “we were” = بودیم (būdīm)
- “you (pl.) were” = بودید (būdīd)
- “they were” = بودَند (būdand)
Negation simply adds a ن before the verb, so “I was not” is نَبودَم (nabūdam), and so forth.
Unlike Arabic and Turkish, which omit the verb is basic present-tense subject-predicate sentences (e.g., “she is my sister”), Persian uses the verb in all circumstances. There is a short form and a long form of the present-tense of the verb “to be” in Persian. Both are totally irregular when compared to the infinitive; the short form is really a set of endings, which I’ll attach to the word خوب (khūb, “good” or “fine,” the usual answer to the question هالِ سُما؟ hāl-i shumā, “how are you”):
- “I am fine” = خوبَم (khūbam)
- “you (sing.) are fine” = خوبی (khūbī)
- “he/she/it is fine” = خوب است (khūb-ast)
- “we are fine” = خوبیم (khūbīm)
- “you (pl.) are fine” = خوبید (khūbīd)
- “they are fine” = خوبَند (khūband)
The short and long forms are basically interchangeable, but the long form is more emphatic and is used when the existence of a thing is what is being stressed:
- “I am” = هَستَم (hastam)
- “you (sing.) are” = هَستی (hastī)
- “he/she/it is” = هَست (hast)
- “we are” = هَستیم (hastīm)
- “you (pl.) are” = هَستید (hastīd)
- “they are” = هَستَند (hastand)
Negating the present tense of “to be” almost always takes the long form, but you change the stem from هَست to نیست (nīst). So, “I am not fine” would be خوب نیستَم (khūb nīstam), etc.
There is an archaic form of the present tense that uses the present stem of بودَن, which is باش (bāsh), so می باشَم (mī bāsham) would be “I am.” However, in most (all?) dialects of modern Persian the only time you should encounter the باش stem is in the subjunctive mood, expressing what should or could be rather than what is.
Persian Word a Day 