tóudēng zài zhèlǐ.

Because 在 is doing the job of is located at / is in / is at.

In Mandarin, when you say where something is, the normal pattern is:

thing + 在 + place

So:

头灯在这里。 = The headlight is here.

You normally do not say 头灯是这里.

A sentence with 是 can appear in special emphatic patterns, such as:

头灯是在这里，不是在那里。
The headlight is here, not there.

But for a plain location statement, 在 is the key word, not 是.

Here, 在 means to be at / to be in / to be located at.

So the sentence structure is:

• 头灯 = the headlight
• 在 = is located
• 这里 = here

Together: The headlight is here.

A useful point: 在 can behave differently in different sentences. For example:

• 头灯在这里。 — 在 is the main verb: is here
• 我在这里等你。 — 在 introduces location before another verb: I’m waiting for you here

So in your sentence, it is best understood as the main location verb.

This is the normal Mandarin order for saying where something is:

subject / thing + 在 + location

Examples:

• 书在桌子上。 — The book is on the table.
• 车在外面。 — The car is outside.
• 头灯在这里。 — The headlight is here.

English uses to be + location. Mandarin often uses 在 + location instead.

A very literal gloss would be something like:

headlight + at + here

That sounds odd in English, but it is completely natural in Chinese.

Mandarin does not have articles like a, an, and the.

So 头灯 by itself can mean something like:

• the headlight
• a headlight
• sometimes even headlights

The exact meaning comes from context.

If you want to be more specific, you can add other words:

• 一个头灯 — a headlight
• 这个头灯 — this headlight
• 那个头灯 — that headlight

But in a simple sentence like 头灯在这里。, leaving the noun bare is very normal.

Mandarin nouns usually do not change form for singular vs. plural the way English nouns do.

So 头灯 can be singular or plural depending on context.

In this sentence, it will usually be understood as the headlight or the headlights, depending on the situation. If you need to be clear, you can add a number:

• 一个头灯在这里。 — One headlight is here.
• 两个头灯在这里。 — Two headlights are here.

English learners often look for an -s ending, but Mandarin normally does not work that way.

Also, 们 is not used for most inanimate objects like 头灯.

Yes. 这里 and 这儿 both mean here.

So these are both natural:

• 头灯在这里。
• 头灯在这儿。

Very roughly:

• 这里 sounds a bit more neutral / standard
• 这儿 is more colloquial, and especially common in northern speech

Both are very common and both are correct.

The pinyin is:

tóudēng zài zhèlǐ

Tones:

• 头 — second tone: tóu
• 灯 — first tone: dēng
• 在 — fourth tone: zài
• 这 — fourth tone: zhè
• 里 — third tone: lǐ

A natural rhythm is:

tóu-dēng | zài | zhè-lǐ

A few pronunciation tips:

• 头灯: let tóu rise, then dēng stay high and level
• 在: short, clear falling tone
• 这里: zhè falls; lǐ is a third tone, which may sound low and slightly dipping in normal speech

You do not need to over-pronounce every tone dramatically. Smooth, connected speech is better than sounding overly mechanical.

They are similar, but the focus is different.

头灯在这里。
Focus: the location of a specific or already relevant headlight
= The headlight is here.

这里有头灯。
Focus: existence/presence
= There is a headlight here.

So:

• Use X 在这里 when you are saying where something is.
• Use 这里有 X when you are saying that something exists or is present here.

This is a very important distinction in Mandarin.

Yes, 头灯 is a compound noun made from:

• 头 = head
• 灯 = lamp / light

Chinese very often builds words this way.

Depending on context, 头灯 can refer to a headlight or a headlamp. The exact meaning depends on what situation you are talking about.

So even though you can see the parts inside the word, it functions as one vocabulary item in the sentence:

头灯在这里。