Learn Hardcore Russian: Иногда слова ранят сердце.

слова – nominative plural of слово (a word). Nominative is the “dictionary form” used for the subject of the sentence.
сердце – accusative singular of сердце (heart). Accusative is used for the direct object, the thing being affected by the action.

So the literal structure is:
(Иногда) слова (subject) ранят (verb) сердце (object).

Why does сердце look the same in nominative and accusative?

Because сердце is a neuter, inanimate noun, and for that group the nominative and accusative singular forms are identical.

Nominative singular: сердце
Accusative singular: сердце

If сердце were animate (a living being), its accusative would usually match the genitive, but inanimate nouns (like objects, abstract things) often keep the nominative form in the accusative.

What exactly does слова mean here: words, the words, or some words?

Russian has no articles (a, an, the), so слова on its own can correspond to any of:

words
the words
some words
certain words

Context decides which English article (if any) you use. In a general statement like this, English normally says words or sometimes words without any article, or sometimes words can hurt your heart.

Why is it ранят and not ранит?

The verb must agree with its subject in number and person.

Subject: слова – plural (they)
Verb: ранить – они ранят (they wound / they hurt)

Conjugation of ранить in the present tense:

я раню – I hurt
ты ранишь – you (sg) hurt
он / она / оно ранит – he / she / it hurts
мы раним – we hurt
вы раните – you (pl / formal) hurt
они ранят – they hurt

Since слова = они (they), we must use ранят.

What aspect is ранить, and what nuance does that give the sentence?

Ранить here is imperfective. Imperfective aspect is used for:

general truths
repeated or habitual actions
ongoing or “open-ended” situations

So Иногда слова ранят сердце is a general statement:
Sometimes words (in general) hurt the heart.

The usual perfective partner is поранить (to wound/hurt once, to end up hurting). For a specific one-time event, you might have something like:
Его слова поранили моё сердце. – His words wounded my heart (on that occasion).

Why is it ранят, not ранятся? What does the -ся ending do?

The -ся ending usually makes a verb reflexive or changes its meaning (e.g. “to wash oneself,” “to be afraid,” “to fight,” etc.).

ранить – to wound / hurt (someone or something)
раниться – to get wounded / hurt oneself (physically), often by accident

In this sentence, words are hurting something else (the heart), not themselves, so we use the non‑reflexive form ранят.
Иногда слова ранят сердце. – Sometimes words hurt the heart.
Он поранился ножом. – He hurt himself with a knife. (reflexive)

Can Иногда go in other positions, like in English “words sometimes hurt the heart”?

Yes. Иногда is an adverb and can move around a bit without changing the core meaning:

Иногда слова ранят сердце. – Sometimes words hurt the heart.
Слова иногда ранят сердце. – Words sometimes hurt the heart.
Слова ранят сердце иногда. – Words hurt the heart sometimes. (possible, but less neutral; can sound more “afterthought-like” or poetic)

The most natural neutral version is usually one of the first two, and starting with Иногда is very common when you want to stress “sometimes” as the frame for the whole statement.

Could it be сердца instead of сердце? What would Иногда слова ранят сердца mean?

Yes, сердца is also possible, but it changes the meaning slightly:

Иногда слова ранят сердце. – Sometimes words hurt the heart / your heart (heart as a singular, often in a more abstract or “someone’s heart” sense).
Иногда слова ранят сердца. – Sometimes words hurt hearts (many hearts, plural).

The singular is more general/abstract and is usually what you want if you mean “one’s heart” in a figurative sense. The plural is also grammatical, but sounds more like “many people’s hearts” at once.

Why is there no word for your before сердце? In English we might say “hurt your heart.”

Russian often omits possessive pronouns (мой, твой, его, etc.) when ownership is clear from context or when talking about body parts, feelings, or something closely connected with a person.

So сердце here can be understood as:

one’s heart
your heart
a person’s heart

without needing твоё or ваше. If you want to be explicit, you can say:

Иногда слова ранят твоё сердце. – Sometimes words hurt your heart.
But in a general proverb-like statement, just сердце is more natural.

Is слова always plural? I’ve seen слово and слов as well. What’s the difference?

Слово is a bit irregular in its plural and case forms. The most important ones:

Nominative singular: слово – a word
Nominative plural: слова – words
Genitive singular: слова – of the word
Genitive plural: слов – of words

In our sentence слова is nominative plural (subject: words).
The exact same form слова can also be genitive singular in other contexts, but the verb ранят (3rd person plural) tells you that here we need a plural subject.

How do I pronounce the words, especially ранят and сердце?

Stress and approximate pronunciation:

Иногда – i-nag-DA (stress on the last syllable)
слова – sla-VA (stress on the second syllable)
ранят – RA-nyat (stress on the first syllable; я here sounds like “ya”)
сердце – SYER-d-tse (stress on the first syllable; the д is not fully pronounced as a separate consonant, more like SYER-tse)

So the whole sentence sounds roughly like:
i-nag-DA sla-VA RA-nyat SYER-tse.