Basically there are three types of sentences: indicative, negative, and interrogative. The relationship between affirmative and negative sentences is grammatically referred to as polarity. (In English, syntactically, no; morphologically, happy vs. unhappy; and lexically, high vs. low represent polarity.

Indicative (also referred to as declarative, sometimes, affirmative):

سارا قلم دارد
sara qalam dārad
‘Sara has (a) pen.’

The Negative is constructed by prefixing the verb with نـ; as in, دارم → ندارم:

سارا قلم ندارد
sara qalam nadārad
‘Sara does not have (a) pen.’

Before ast ‘is’ and hast ‘it is; it exists’ نـ becomes نیـ and the alef of ast and the /h/ of hast are contracted, hence, both become نیست ‘is not’:

این خوب نیست in xub nist ‘This is not good.’ ← این خوب است in xub ast ‘This is good.’

خوب نیستم xub nistam ‘I am not fine/good.’ ← خوب هستم xub hastam ‘I am fine/good.’

Interrogative:

In Persian, the interrogative and indicative sentences are constructed the same. It is only through the intonation and articulation that the listener recognizes a sentence as being an indicative or interrogative. (In English the word order changes.)

سارا قلم دارد؟
sara qalam dārad?
‘Does Sara have a pen?’

In the indicative, the tone falls at the end of the sentence, while in the interrogative, it rises. If one of the interrogative pronouns (who, what, when, where, why, which, how) is present, it takes the stress, and, like the indicative, the voice inflection falls at the end of the sentence. Compare the following two sentences:

. سارا قلم دارد
sara qalam dārad.
‘Sara has a pen/some pens.’

سارا قلم دارد؟
sara qalam dārad?
‘Does Sara have a pen/some pens?’

کی قلم دارد؟
ki qalam dārad?
‘Who has a pen?’

With the rise of the voice inflection falling on any of the grammatical units the meaning of the sentence changes. Compare the following sentences:

سارا قلم دارد؟
sara qalam dārad?
‘Sara has a pen? (Never! She is not the type!)’

سارا قلم دارد؟
sara qalam dārad?
‘Sara has a pen? (I believe she only has a pencil.)’

سارا قلم دارد؟
sara qalam dārad?
‘[Are you sure that] Sara has a pen? (She usually doesn’t.)’

In formal Persian (and occasionally in colloquial Persian) sometimes an interrogative sentence begins with the interrogative particle آیا. This prepares the listener:

آیا سارا قلم دارد؟
āyā sara qalam dārad?

The syntactic function of آیا is very close the “tell me” in the sentence, “Tell me, are you really happy?” which is a sentence opener that prepares the listener for a question to be posed.

To sum up, an interrogative sentence may be indicated by (1) the intonation, (2) by the prepositional particle آیا, or (3) by using one of the interrogative pronouns:

کتاب را خواندی
ketāb rā xāndi
‘You read the book.’ Indicative,

کتاب را خواندی؟
ketāb rā xāndi?
‘Did you read the book?’ Interrogative,

آیا کتاب را خواندی؟
āyā ketāb rā xāndi?
‘Did you read the book?’ Interrogative,

کِی کتاب را خواندی؟
key ketāb rā xāndi?
‘When did you read the book?’ Interrogative,

In the last sentence, the interrogative pronoun takes the stress, and, like the indicative, the voice inflection falls at the end of the sentence, while it rises in the other two interrogative sentences.