Takot siyang mapansin.

Because Filipino often uses a predicate-first word order.

In this sentence:

• takot = the predicate (afraid)
• siya = the topic/subject (he/she)

So Takot siya is the normal way to say He/She is afraid.

An English-like order such as Siya ay takot is possible, but it sounds more formal, marked, or literary. In everyday speech, Takot siya is more natural.

Siyang is siya + -ng.

• siya = he/she
• -ng = a linker

So siyang mapansin means something like:

• he/she to be noticed
• he/she who may be noticed
• more naturally in full sentence context: he/she is afraid of being noticed

The linker connects siya to the following word or phrase, which here is mapansin.

Filipino linkers have two common forms:

• -ng after words ending in a vowel
• na after many words ending in a consonant

Since siya ends in a vowel, the linker becomes attached as -ng:

• siya + -ng = siyang

Compare:

• mabait na bata
• batang mabait

Both na and -ng are forms of the same linker, but which one appears depends on the sound of the word before it.

Here, mapansin means to be noticed or to get noticed.

It comes from pansin, which is related to notice/attention.

In this sentence, the idea is that the person is afraid that other people will notice him or her. So the natural sense is:

• be noticed
• get noticed

This is the meaning that fits the sentence best.

Because of how the sentence is structured.

The whole sentence means that siya is the person who is afraid, and what he or she is afraid of is mapansin. So the natural interpretation is:

• He/She is afraid of being noticed

In other words:

• takot describes siya
• mapansin tells us what the fear is about

So siya is both:

• the experiencer of the fear, and
• the person who would be noticed

Because siya is the pronoun used for the subject/topic here.

• siya = he/she
• niya = his/her, or sometimes by him/her, depending on the sentence

In Takot siyang mapansin, the sentence is about he/she as the main topic, so siya is the correct form.

If you used niya, the structure and meaning would change. For example:

• Ayaw niyang mapansin siya
  = He/She doesn’t want himself/herself to be noticed or more naturally He/She doesn’t want to be noticed

So siya and niya are not interchangeable.

Yes. That is also natural.

The difference is roughly this:

• Takot siya = He/She is afraid
• Natatakot siya = He/She is feeling afraid / is afraid

So:

• Takot siyang mapansin
• Natatakot siyang mapansin

Both can mean almost the same thing in English.

A useful way to think about it:

• takot is more like a predicate adjective or state
• natatakot is a verb form

In many everyday situations, either one works.

No. Siya can mean either:

• he
• she

Filipino pronouns do not usually mark gender the way English pronouns do. So you need context to know whether the person is male or female.

No word is missing.

The linker is already present in siyang:

• siya + -ng + mapansin

So the connection is already built into the sentence.

You may sometimes see structures with na, but in this sentence the natural form is the linked one:

• Takot siyang mapansin

That is a compact and very normal Filipino way to say it.

It can be rephrased, but it is less natural in ordinary speech.

Compare:

• Takot siyang mapansin — most natural
• Siya ay takot mapansin — possible, but more formal or stiff

Filipino usually prefers the predicate-first version in simple statements like this, especially in conversation. So a learner should treat Takot siyang mapansin as the standard pattern.