Одного кивка было достаточно, и мы поняли друг друга без слов.

Because достаточно normally takes the genitive case: you are literally saying something like enough of one nod.

So:

• один кивок = one nod (nominative)
• одного кивка = of one nod / one nod’s worth (genitive)

This is very common with words of quantity or sufficiency:

• достаточно времени = enough time
• много людей = many people
• мало денег = little money

In this sentence, одного is the genitive form of один, and кивка is the genitive singular of кивок.

This is an impersonal construction. In Russian, expressions like было достаточно, было трудно, было ясно, было холодно often use neuter singular было.

So the structure is not really the nod was sufficient in the same way English might phrase it. It is more like:

• Одного кивка было достаточно
  = One nod was enough / It was enough to have one nod

That is why Russian uses было, the usual past-tense form in impersonal sentences.

Кивок means a nod — a single nodding movement of the head.

It refers to one completed gesture. That is why it fits very naturally here: one nod was enough for mutual understanding.

Compare:

• кивок = a nod, one discrete gesture
• кивать = to nod
• кивание = nodding, the action/process of nodding

So одного кивка is specifically one nod, not nodding in general.

Поняли is the perfective past of понять, and it shows a completed result: they successfully understood each other.

• мы поняли друг друга = we understood each other
• мы понимали друг друга = we understood each other / were understanding each other (more ongoing, habitual, or descriptive)

In this sentence, the idea is that after that one nod, the understanding happened as a completed event. So the perfective поняли is the natural choice.

Друг друга is the standard Russian way to say each other.

It behaves as a reciprocal expression:

• мы поняли друг друга = we understood each other
• они видят друг друга = they see each other
• мы помогаем друг другу = we help each other

A useful rule:

• the second part changes case depending on what the verb requires
• the first part often stays друг

Here, понять кого? takes the accusative, so we get:

• друг друга

For masculine animate-type forms, accusative looks like genitive, which is why друга may look surprising.

It is functioning as the accusative here, because понять takes a direct object:

• понять кого? что? = to understand whom? what?

So in мы поняли друг друга, друг друга is the object of the verb.

However, the form друга looks the same as genitive because with masculine animate nouns/pronoun-like forms, accusative often matches genitive. That is why learners often find it confusing.

Other cases look different:

• друг другу = to each other
• друг с другом = with each other
• друг о друге = about each other

Because без takes the genitive case, and here Russian uses the plural expression без слов.

• без
  • genitive
• слово → genitive plural слов

So:

• без слов = without words
• без слова = without a word

Both expressions exist, but they are not always identical in feel:

• без слов often means without speaking, wordlessly
• без слова often means without saying a word

In this sentence, без слов sounds very natural because it emphasizes silent mutual understanding.

Yes. Russian word order is flexible, and this sentence could be rearranged without changing the basic meaning.

For example:

• Было достаточно одного кивка, и мы поняли друг друга без слов.
• Мы без слов поняли друг друга.
• Мы поняли друг друга без слов.

The actual order Одного кивка было достаточно puts emphasis early on one nod. It gives the sentence a slightly more expressive, literary feel.

So the word order here is not random — it helps highlight what was enough.

Because и here joins two independent clauses:

1. Одного кивка было достаточно
2. мы поняли друг друга без слов

Since both parts have their own grammatical basis, Russian normally puts a comma before и.

This is similar to English:

• One nod was enough, and we understood each other without words.

If и joined only words or simple parts of one clause, there would usually be no comma. But here it connects two full clauses, so the comma is correct.