Learn Hardcore Russian: Мой палец болит, потому что я случайно порезал его ножом. - My finger hurts because I accidentally cut it with a knife.

У меня болит палец (literally “At me the finger hurts”) is the most common, neutral way to say “My finger hurts.” This is what people usually say when complaining about pain.
Мой палец болит is also correct, but it emphasizes ownership: my finger (as opposed to someone else’s).
- It might appear in a contrast: У него всё нормально, а мой палец болит. – “His is fine, but my finger hurts.”

So in casual conversation about symptoms, У меня болит палец or just Палец болит are more typical, but Мой палец болит is not wrong.

What is the difference between болит, мне больно, and я болею?

They look similar but are used differently:

болит – “hurts / aches” (a body part is the subject):
- У меня болит палец. – “My finger hurts.”
- У меня болят пальцы. – “My fingers hurt.”
- Forms used in this “ache” meaning are mostly болит (singular) and болят (plural).
мне больно – “it hurts (me) / I feel pain” (focus on your feeling of pain, not which organ):
- Мне больно. – “It hurts.”
- Мне больно двигать пальцем. – “It hurts to move my finger.”
я болею – “I am ill / I’m sick” (general illness, not just one body part):
- Я болею. – “I’m sick.”
- Он часто болеет. – “He is often ill.”

In your sentence, you need “a body part hurts,” so палец болит is the right choice.

Can I just say Палец болит instead of Мой палец болит? Will it still mean “my finger hurts”?

Yes. In context, Палец болит will normally be understood as “My finger hurts,” especially if you are talking about your own health.

Russian often omits possessive pronouns for body parts when it’s obvious whose they are:

У меня болит голова. – “My head hurts.” (literally “At me hurts head.”)
Нога болит. – “(My) leg hurts.”

Мой палец болит is more explicit, but Палец болит is perfectly natural and very common.

What exactly does потому что mean, and can it be written differently (like потому, что)?

In your sentence, потому что is a single conjunction meaning “because.”

Мой палец болит, потому что я случайно порезал его ножом.
“My finger hurts because I accidentally cut it with a knife.”

Literally, потому comes from “for that reason,” and что is “that,” but together as потому что they function just like English because.

The spelling потому, что (with a comma in between) occurs in other constructions where что is a relative pronoun, not a conjunction. That’s a different grammar situation; in your example it’s simply потому что (no internal comma).

Why do we use порезал and not резал? What’s the nuance?

The verbs differ in aspect and nuance:

резать (imperfective) – “to cut” in general, ongoing or repeated action.
порезать (perfective, with prefix по-) – “to cut (once / as a completed action),” often implying a result like a wound.

In your context, you’re describing one completed action that caused an injury, so Russian naturally uses the perfective past:

я (случайно) порезал его ножом – “I (accidentally) cut it with a knife” (and now there is a cut).

If you said резал, it would sound more like describing the process of cutting in general (e.g. “I was cutting something”), not the single finished accident that led to pain.

Why is it порезал and not порезала / порезали?

Порезал is the masculine singular past tense form of порезать.

masculine singular: порезал (я, он)
feminine singular: порезала (я, она)
neuter singular: порезало (оно – very rare in this context)
plural: порезали (мы, вы, они)

The verb in the past agrees with the gender and number of the subject:

Я порезал палец. – said by a man.
Я порезала палец. – said by a woman.
Мы порезали палец (e.g. “we cut a finger” – plural subject).

So in your sentence, я порезал implies the speaker is male.

Could I use the reflexive form порезался instead of порезал его ножом? How would the meaning change?

Yes, you can, and it will sound very natural:

Мой палец болит, потому что я случайно порезался ножом.

Порезаться is the reflexive form: “to cut oneself / get cut.”

Compare:

я порезал его ножом – “I cut it with a knife.”
- Grammatically, его is a direct object (“I cut it”).
- Context tells us его = палец.
я порезался ножом – “I cut myself with a knife.”
- The reflexive -ся marks that the subject and object are the same person.
- This is what Russians very often say for accidents like this.

So порезался ножом is actually more idiomatic for “I accidentally cut my finger with a knife.”

What does его refer to here? Could it mean “his finger” instead of “my finger”?

In this sentence:

Мой палец болит, потому что я случайно порезал его ножом.

the most natural interpretation is:

его = “it,” referring back to палец (finger).

So: “My finger hurts because I accidentally cut it with a knife.”

Could it mean “his finger”? Grammatically, его can mean “him / his / its,” but in this specific context:

You already said Мой палец (“my finger”).
If you wanted to say “I cut his finger,” you’d normally say я порезал ему палец ножом (“I cut his finger with a knife”), not порезал его.

So, by default, native speakers will understand его here as “it,” i.e. my finger.

What case is его in, and why is that form used?

Here его is in the accusative case, functioning as the direct object of порезал:

Nominative (subject): он – “he / it (masc.)”
Accusative (direct object): его – “him / it”

Since я порезал кого? что? (“I cut whom / what?”) – you answer that question with the accusative, so его is the correct form.

Note: for masculine inanimate nouns and pronouns, the accusative often looks like the genitive, which is why его also serves as the genitive form.

Why is ножом in the instrumental case, and could I just say нож instead?

Ножом is the instrumental case of нож (“knife”).

Russian uses the instrumental case to show the tool or means by which an action is done, often without a preposition:

Я пишу ручкой. – “I write with a pen.”
Он открыл дверь ключом. – “He opened the door with a key.”
Я порезал его ножом. – “I cut it with a knife.”

So ножом answers “with what?” and must be instrumental.

You cannot use nominative нож here; я порезал его нож would mean something like “I cut his knife” (and is still wrong grammatically in this form). To express the instrument “with a knife,” you need ножом.

Where can I place случайно in the sentence? Does its position change the meaning?

Случайно means “accidentally.” Russian word order is flexible, so several positions are possible, with only slight differences in emphasis:

Я случайно порезал его ножом. – neutral, very common.
Я его случайно порезал ножом. – a bit more emphasis on the fact that it was accidentally cut.
Я ножом случайно порезал его. – less common, can sound slightly marked/focused in speech.
Случайно я порезал его ножом. – emphasizes the accidental nature more strongly, often stylistic.

All of these still mean essentially “I accidentally cut it with a knife.”
The default, most natural version is Я случайно порезал его ножом.

Is it necessary to say его? Can I say …потому что я случайно порезал палец ножом instead?

Yes, you can, and this is actually more typical:

Мой палец болит, потому что я случайно порезал палец ножом.
Even more natural: У меня болит палец, потому что я его случайно порезал ножом.
or
У меня болит палец, потому что я случайно порезал палец ножом.

Common options:

порезал палец ножом – explicitly repeats the noun палец.
его порезал ножом – uses его to avoid repetition.

Both are correct. Using его in your original sentence is fine, but many speakers would simply repeat палец, especially if there is no risk of sounding clumsy or unclear.

If I wanted a fully idiomatic Russian version of “My finger hurts because I accidentally cut it with a knife,” what are some natural variants?

Very natural-sounding options include:

У меня болит палец, потому что я случайно порезался ножом.
– “My finger hurts because I accidentally cut myself with a knife.”
(reflexive; very idiomatic)
У меня болит палец, потому что я его случайно порезал ножом.
– “My finger hurts because I accidentally cut it with a knife.”
Палец болит, я его случайно ножом порезал.
– More colloquial, as in spoken language.

Your original sentence is correct; these are just variants that sound especially natural in everyday Russian.