يبدأ بعد ثانية.

يبدأ is the imperfect form of the verb بدأ, meaning to begin / to start.

Its form is:

• 3rd person
• masculine
• singular

So it can mean:

• he begins / starts
• it begins / starts

The exact subject depends on context.

Because Arabic often leaves the subject unstated when the verb already shows it.

In يبدأ, the verb itself already tells you that the subject is 3rd person masculine singular. So Arabic does not need to add a separate pronoun unless it is needed for emphasis or clarity.

There is an understood pronoun here, roughly هو, but it is not written.

Yes. By itself, يبدأ can mean either he starts or it starts.

Arabic verbs do not make a separate verb form for he versus it. Both use the same 3rd person masculine singular form if the noun is grammatically masculine.

So the sentence could refer to:

• a man
• a masculine noun
• something translated as it in English

Only context tells you which one is meant.

Formally, يبدأ is in the imperfect, which can cover:

• present
• ongoing action
• near future
• general statements

In this sentence, because of بعد ثانية, the meaning is understood as future: the action will start one second later.

If you want to make the future more explicit, Arabic can also say:

• سيبدأ بعد ثانية

That is closer to it will start after a second.

بعد means after here.

In this sentence, it introduces a time expression:

• بعد ثانية = after a second

So it tells you when the starting happens.

Here ثانية means a second as a unit of time.

So بعد ثانية means after a second.

A useful point: ثانية can also mean second in the ordinal sense, as in the second item, but that is not the natural meaning here. In this sentence, it is understood as the time unit.

Because the noun ثانية meaning second is grammatically feminine in Arabic.

That is just part of the word itself. It is not feminine because of anything in English; it is simply the grammatical gender of this Arabic noun.

So:

• ثانية = one second
• ثانيتان / ثانيتين = two seconds
• ثوانٍ = seconds

Arabic has no separate word for a or an.

Instead, indefiniteness is usually shown by the noun being without ال.

So:

• ثانية = a second
• الثانية = the second or the second one, depending on context

That is why بعد ثانية naturally means after a second.

Yes, in fully vocalized formal Arabic, the word after بعد would be in the genitive.

So the fully vowelled form would be:

• يبدأُ بعدَ ثانيةٍ

But in normal Arabic writing, short vowels and case endings are usually not written, so you normally just see:

• يبدأ بعد ثانية

That is completely normal.

Because verb-first word order is very common in Arabic.

So a sentence beginning with the verb, like:

• يبدأ بعد ثانية

is perfectly natural.

Arabic can also change word order for emphasis, but the verb-first pattern is one of the most common basic sentence patterns.

A careful pronunciation is roughly:

• yabda'u baʿda thāniyah

A few notes:

• يبدأ contains a hamza: yabdaʾu
• بعد has the deep consonant ع, often written baʿda
• ث in ثانية is like th in think

If fully vocalized grammatically, it would be:

• yabdaʾu baʿda thāniyatin

But in normal pause at the end of the sentence, learners often hear something closer to:

• yabdaʾ baʿda thāniyah

Yes. بعد ثانية واحدة is also correct and can sound a little more explicit, because واحدة clearly reinforces one second.

So both are possible:

• بعد ثانية
• بعد ثانية واحدة

The shorter version is already perfectly understandable.