Learn Hardcore Russian: Днём датчик иногда реагирует на моё движение, и лампа включается просто так. - During the day the sensor sometimes reacts to my movement, and the lamp turns on for no reason.

The base noun is день (day).
днём is the instrumental singular form of день.
In Russian, the instrumental case is often used adverbially to express “at / during (a time of day)”:
- днём – in the daytime
- ночью – at night
- утром – in the morning
- вечером – in the evening

So Днём датчик… literally is “By day / During the day, the sensor…”, but in natural English: “In the daytime, the sensor…”

I sometimes see днем without ё. Is that the same word as днём?

Yes, днем and днём are the same word.

The correct, fully marked form is днём (with ё),
But in many printed texts, Russian ё is written simply as е, so you get днем.

Pronunciation is always with ё /dnyom/, not /dnem/. Writing е instead of ё is extremely common in Russian, especially in newspapers, books, and on the internet.

Could you explain the structure датчик иногда реагирует на моё движение? What case is used after на and why?

The structure is:

реагировать на что? – “to react to something”
After на with this verb, you use the accusative case.

In the sentence:

движение is a neuter noun.
Its nominative and accusative singular forms are the same: движение.
моё is the neuter singular form of мой and also goes into the accusative, agreeing with движение.

So:

на моё движение = “to my movement” (literally “onto my movement”), with движение in the accusative because the pattern is реагировать на + accusative.

Why is it моё движение and not мой движение or моим движением?

Because движение is:

neuter gender,
singular,
accusative case.

The possessive мой must agree with the noun in gender, number, and case:

Masculine nominative: мой стол
Feminine nominative: моя книга
Neuter nominative/accusative: моё окно, моё движение
Instrumental neuter: моим движением, etc.

Here, after на, we need accusative, and neuter accusative singular for движение looks like the nominative: движение. The matching form of мой is моё, so: на моё движение.

Why is движение singular (моё движение) if the motion happens many times?

In Russian, you often use a singular abstract noun to talk about a type of action or event that can happen many times.

моё движение here means “my movement” in a general, non‑countable sense:
whenever I move in front of the sensor.

You could theoretically say на мои движения (“to my movements”), but that would sound more like separate, countable movements being considered as individual events. In this context, на моё движение as a general phenomenon is more natural.

Why is there a comma before и лампа включается просто так?

The sentence consists of two independent clauses joined by и (“and”):

Днём датчик иногда реагирует на моё движение – “In the daytime the sensor sometimes reacts to my movement,”
(и) лампа включается просто так – “(and) the lamp turns on for no reason.”

In Russian, when и connects two full clauses (each has its own subject and verb), you normally use a comma before и:

…реагирует на моё движение, и лампа включается…

If you had a shared subject and only two verbs, you might not need a comma, e.g.:

Датчик иногда реагирует и включает лампу. – one subject, two verbs.

What is the difference between включается and включает here?

включаться (imperfective, reflexive) – “to turn on, to switch on (by itself, or becomes switched on)”
включать (imperfective, non‑reflexive) – “to turn on something (you actively switch it on).”

In the sentence:

лампа включается = “the lamp turns on” (it becomes on; the process happens to it).

If you said:

датчик включает лампу – “the sensor turns the lamp on.”

That would make датчик the doer that actively switches the lamp on. Using включается focuses on the lamp’s state changing, not on who switches it on. It fits well with an automatic system.

What does просто так mean here? Is it literally “simply like this”?

Literally, просто = “simply/just” and так = “like this/that”.
But the fixed phrase просто так is an idiom.

In this context, просто так means:

“for no good reason,” “for nothing,” “for no particular reason,” “just because.”

So лампа включается просто так is best understood as:

“the lamp just turns on for no reason,”
not “the lamp turns on simply like this.”

What tense/aspect are реагирует and включается, and why are both in the present tense even though the lamp turns on after the sensor reacts?

Both verbs are:

present tense,
imperfective aspect.

In Russian, the present imperfective is used for:

regular, repeated, or habitual actions.

So датчик иногда реагирует … и лампа включается… describes what happens from time to time in general, not one specific instance.

The fact that the lamp turns on after the reaction is understood from logic and context, not from tense. Russian doesn’t need a special tense contrast here; both parts are part of the same typical scenario:

Whenever this happens, the sensor reacts, and then the lamp comes on.

Where can I put иногда in this sentence? Are other positions possible?

Yes, иногда (“sometimes”) is quite flexible. Some natural variants:

Днём датчик иногда реагирует на моё движение… (original)
Днём иногда датчик реагирует на моё движение…
Иногда днём датчик реагирует на моё движение…
Датчик иногда днём реагирует на моё движение… (less typical, but possible in spoken language).

The main rule: keep иногда close to the verb or clause it modifies. The differences are mostly about subtle emphasis:

Starting with Иногда highlights how often it happens (“Sometimes, during the day…”).
Starting with Днём highlights when it happens (“During the day, the sensor sometimes…”).

Could I say днём датчик иногда реагирует на меня instead of на моё движение? Would that change the meaning?

You can say реагирует на меня, but the nuance changes:

реагирует на моё движение – reacts to my movement, i.e., to the fact that I move (this fits the idea of a motion sensor).
реагирует на меня – reacts to me (as a person), more like it somehow notices me specifically, not just any movement.

For a typical motion sensor, на моё движение (or simply на движение) is more precise and natural.

Is датчик masculine, feminine, or neuter, and how would I refer back to it with pronouns?

датчик is masculine.

Clues:

Ending in a consonant: -чик – usually masculine.
In the sentence: датчик реагирует – the verb is just реагирует (3rd person masculine or neuter), but context and the word’s typical gender tell us it’s masculine.

If you refer back to датчик, you use masculine pronouns:

он – he/it
его – his/its
с ним – with it

Example:

Этот датчик иногда срабатывает неправильно. Он реагирует на моё движение даже днём.
“This sensor sometimes works incorrectly. It reacts to my movement even in the daytime.”

Could the sentence also start with Иногда instead of Днём? Would that change the emphasis?

Yes, you can say:

Иногда днём датчик реагирует на моё движение, и лампа включается просто так.

This is grammatical and natural. The difference:

Днём датчик иногда реагирует… – puts time first: “In the daytime, the sensor sometimes reacts…”
Иногда днём датчик реагирует… – puts frequency first: “Sometimes, during the day, the sensor reacts…”

Both ideas are present in both versions, but the initial word gets slightly more emphasis in Russian word order.