تبدأ الحصة الآن.

Because the subject الحصة is a feminine singular noun.

In Arabic, present-tense verbs agree with the subject in gender.

• تبدأ = she starts / it starts for a feminine singular subject
• يبدأ = he starts / it starts for a masculine singular subject

Since حصة is feminine, the sentence uses تبدأ.

الحصة usually means the class, the lesson, or the period, depending on context.

In a school context, تبدأ الحصة الآن most naturally means something like:

• The class starts now
• The lesson begins now
• The period starts now

The word comes with الـ because it is definite: the class, not just a class.

Arabic very often uses verb–subject order, especially in straightforward statements.

So:

• تبدأ الحصة الآن = literally Starts the class now
• Natural English: The class starts now

This word order is completely normal in Arabic.
You could also say الحصة تبدأ الآن, which is also understandable and natural, but تبدأ الحصة الآن is a very common verbal sentence pattern.

الآن means now.

It is an adverb of time, and in Arabic it often comes after the verb and subject:

• تبدأ الحصة الآن = The class starts now

Arabic word order is somewhat flexible, so you may also see:

• الآن تبدأ الحصة = Now the class starts
• الحصة تبدأ الآن = The class starts now

But the given order is very standard.

Formally, تبدأ is a present-tense verb. In Arabic, the present tense can also express the near future depending on context.

So تبدأ الحصة الآن means something happening now / right now.
If the context were different, تبدأ could also mean starts in a more general sense.

For a clearer future meaning, Arabic often uses سـ or سوف:

• ستبدأ الحصة = The class will begin
• سوف تبدأ الحصة = The class will begin

But with الآن, the meaning is clearly now.

A careful pronunciation is roughly:

tabda'u

Important points:

• تَ = ta
• بْ = b with no vowel after it
• دَ = da
• أُ = a short u sound at the end

So it sounds approximately like tab-da-u, though in natural speech the last part is smoother.

Also, the أ in تبدأ is a real hamza, so there is a slight break before the final vowel.

The final ة is called taa marbuuTa (ة).

In pause, الحصة is usually pronounced al-ḥiṣṣa.
That final ة sounds like -a when you stop there.

If the word is followed by certain grammatical endings in connected speech, it can sound like t. For example in constructions like:

• حصةُ اللغة العربية = the Arabic-language class

But in the sentence تبدأ الحصة الآن, you normally hear it as al-ḥiṣṣa.

Because Arabic verbs already contain subject information.

In English, you need it starts.
In Arabic, تبدأ already tells you that the subject is feminine singular, so a separate pronoun is not necessary.

The noun الحصة then names what is starting.

So Arabic does not need an extra word equivalent to it here.

This is a verbal sentence (جملة فعلية), because it begins with a verb.

Its parts are:

• تبدأ = the verb
• الحصة = the subject/doer of the action
• الآن = an adverb of time

This is one of the most basic and common Arabic sentence types.

Yes, depending on context.

حصة can refer to several kinds of scheduled periods or allotted portions, so possible translations include:

• class
• lesson
• period
• session

In a school setting, class or lesson is usually best. In another context, session might be appropriate.

It is always grammatically feminine.

That is why it takes feminine agreement:

• تبدأ الحصة = The class starts

If the subject were masculine, the verb would change:

• يبدأ الدرس الآن = The lesson starts now

So the feminine form is not optional here; it comes from the noun itself.

Yes. A fully vowelled version is:

تَبْدَأُ الحِصَّةُ الآنَ

This shows:

• تَبْدَأُ = tabda'u
• الحِصَّةُ = al-ḥiṣṣatu
• الآنَ = al-āna

In normal everyday writing, these short vowels are usually omitted, so you typically see:

تبدأ الحصة الآن