Напёрсток защищает палец.

Напёрсток is the subject: it is the thing doing the action.

Палец is the object: it is the thing receiving the action.

So the structure is:

• Напёрсток = thimble
• защищает = protects
• палец = finger

Literally: Thimble protects finger.

In natural English, we say A/The thimble protects a/the finger.

Because it is in the accusative case, and for many inanimate masculine nouns in Russian, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: палец
• accusative: палец

That is why the direct object still appears as палец.

This is very common in Russian:

• Я вижу стол. = I see the table.
• стол stays стол in the accusative because it is masculine singular and inanimate.

Защищает is the 3rd person singular present tense form of защищать.

It means:

• he protects
• she protects
• it protects

Here it means it protects, because наперсток is a thing.

The ending -ет shows that the verb agrees with a singular third-person subject.

Conjugation of защищать in the present tense:

• я защищаю = I protect
• ты защищаешь = you protect
• он/она/оно защищает = he/she/it protects
• мы защищаем = we protect
• вы защищаете = you protect
• они защищают = they protect

The dictionary form is защищать.

That is the imperfective infinitive, usually translated as to protect.

There is also a perfective partner: защитить.

Very roughly:

• защищать = to protect, to be protecting, to protect in a general or repeated sense
• защитить = to protect successfully, to defend, to protect once / to completion

In this sentence, защищает is a normal choice for a general statement:
A thimble protects the finger.

A helpful pronunciation guide is:

nap-YOR-stək zash-chi-SHCHA-yet PA-l’its

A few important points:

• ё in напёрсток is always stressed: напЁрсток
• защищает is stressed on -ща-: защищАет
• палец is stressed on the first syllable: пАлец

More carefully:

• Напёрсток = na-PYOR-stak / na-PYOR-stək
• защищает = za-shchi-SHCHA-yet
• палец = PA-l’its

The cluster щ sounds like a long soft sh sound.

Yes, it is important.

Ё represents the sound yo, and in Russian it is always stressed.

So:

• напёрсток = na-PYOR-stok

In many printed texts, Russians often write е instead of ё, so you may sometimes see наперсток. But the word is still pronounced напёрсток.

For learners, it is very useful to remember the ё, because it tells you both:

• the correct vowel sound
• the stress

Russian does not have articles like English a/an/the.

So Напёрсток защищает палец can mean:

• A thimble protects a finger
• The thimble protects the finger
• A thimble protects the finger

Which one sounds best depends on context.

When translating into English, you add the article that fits the situation. But in Russian, there is no separate word for it.

You usually can change Russian word order, but in this sentence you should be careful.

Normal, neutral order:

• Напёрсток защищает палец.

If you say:

• Палец защищает напёрсток.

that normally means:

• The finger protects the thimble.

Why? Because both наперсток and палец are masculine singular inanimate nouns, and in this sentence their forms do not clearly mark subject vs. object. So word order becomes very important.

In other words, this is one of those cases where Russian is less flexible than learners may expect, because the case endings do not help you distinguish the roles here.

Yes, it could be plural in another sentence, but the singular here is perfectly natural.

Russian often uses the singular when talking about something in a general way:

• Напёрсток защищает палец. = A thimble protects the finger.

This means the kind of thing a thimble does.

If you wanted to talk about fingers more generally, you might also hear something like:

• Напёрсток защищает пальцы. = A thimble protects the fingers.

But the singular works well for a general statement about the body part being protected during sewing.

Напёрсток is in the nominative case, because it is the subject of the sentence.

So the sentence has:

• Напёрсток — nominative singular
• палец — accusative singular
• защищает — 3rd person singular present tense

That is a very common Russian sentence pattern:

[subject in nominative] + [verb] + [direct object in accusative]

In this sentence, protects is the better translation.

Russian present tense can sometimes correspond to either English simple present or present continuous, depending on context. But here the sentence is a general fact about what a thimble does, so:

• Напёрсток защищает палец. = A thimble protects the finger.

Using is protecting would sound more like a specific situation happening right now, and that is not the usual reading here.