أتصل حينها.

In fully vocalized MSA, it is أَتَّصِلُ, pronounced roughly attasilu.

Two useful things to notice:

• The ت is effectively doubled, because the verb has a shadda on it in the full form.
• In normal everyday writing, short vowels and shadda are usually omitted, so you just see أتصل.

So even though it is written simply as أتصل, a learner should read it as أَتَّصِلُ, not atasilu.

Because the verb itself already tells you the subject.

أتصل is the 1st person singular form, so it already means I call / I contact. Arabic often leaves subject pronouns out when the verb makes the subject clear.

So:

• أتصل حينها = I call then / I’ll call then
• أنا أتصل حينها = also possible, but أنا adds emphasis, like I will call then

This is very normal in Arabic.

Formally, أتصل is in the imperfect form, which can express:

• present
• habitual action
• near future
• future, depending on context

So by itself, أتصل can mean things like:

• I call
• I am calling
• I call regularly
• I will call

In this sentence, حينها strongly suggests a time in the future or a later point, so the natural meaning is often I’ll call then.

If you want to make the future completely explicit, you can say:

• سأتصل حينها = I will call then
• سوف أتصل حينها = I will call then with slightly stronger/formal future marking

حينها means then, at that time, or at that point.

It is a time expression, not just a simple connector.

That matters because it is slightly different from ثم:

• ثم = then / and then in the sense of sequence
• حينها = at that time / at that point

So أتصل حينها is more like:

• I’ll call at that time
• I’ll call then

rather than simply and then I call.

The ها is a pronominal suffix attached to حين.

Literally, it is something like its time or that time, but in actual use حينها functions as a set time expression meaning then or at that time.

For learners, the most useful thing is to recognize حينها as a whole expression.

You do not need to think of it as strongly feminine in English translation, even though ها is the same suffix often used for her / it.

It comes from the past-tense verb اتَّصَلَ, from the root و ص ل, which is related to the idea of connecting or reaching.

In MSA, اتصل commonly means:

• to contact
• to call
• to get in touch
• sometimes to be connected

The form in your sentence, أتصل, is the 1st person singular imperfect:

• اتصل = he contacted / he got in touch
• أتصل = I contact / I call / I will call

Yes. In MSA, اتصل usually takes بـ for the person or thing contacted.

Examples:

• أتصل به = I call him / I contact him
• أتصل بها = I call her / I contact her
• أتصل بالشركة = I contact the company

So your sentence is fine on its own because the object is not stated, but if you add one, بـ is normally what you want in standard Arabic.

Yes, you can.

Both are possible:

• أتصل حينها
• حينها أتصل

The difference is mainly one of focus and style:

• أتصل حينها is a very normal verb-first sentence
• حينها أتصل puts more emphasis on then / at that time

So if you want to highlight the timing, starting with حينها works well.

A fully vocalized version is:

أَتَّصِلُ حينَها

A few notes:

• أَتَّصِلُ = the fully vocalized verb
• حينَها = the time expression with حين in the accusative as an adverb of time

In ordinary writing, these short vowels are usually omitted, so you normally just see:

أتصل حينها