В письме была одна опечатка, но из-за неё смысл получился странным.

Because в can take different cases depending on meaning:

• в + accusative = motion into something
• в + prepositional = location in something

Here the sentence means in the letter, not into the letter, so Russian uses the prepositional case:

• письмо → в письме

So В письме была одна опечатка means There was one typo in the letter.

The verb была agrees with опечатка, which is:

• singular
• feminine

So the past tense of быть-type meaning here is feminine singular:

• был = masculine
• была = feminine
• было = neuter
• были = plural

Because опечатка is feminine, you get:

• была одна опечатка

Even though в письме comes first, the verb still agrees with опечатка, not with письме.

Because один behaves like an adjective and must agree with the noun in gender, number, and case.

Опечатка is feminine singular nominative, so:

• masculine: один
• feminine: одна
• neuter: одно

That gives:

• одна опечатка

Опечатка specifically means a typo / misprint—a mechanical writing or typing mistake.

Ошибка is broader and means mistake / error in general.

So:

• опечатка = typo
• ошибка = mistake more generally

In this sentence, опечатка is used because the strange meaning came from a typing or spelling slip, not just any kind of error.

The preposition из-за means because of or sometimes from behind, and it requires the genitive case.

The pronoun referring to опечатка must therefore be in the feminine genitive form:

• nominative: она
• genitive after из-за: неё

So:

• из-за неё = because of it / because of her

Here it means because of it, referring to опечатка.

After many prepositions, Russian third-person pronouns gain an initial н:

• у него
• к ней
• с ними
• из-за неё

So although the pronoun is basically её, after a preposition it becomes неё.

This is a normal grammar pattern, and learners just have to get used to it.

Literally, it is something like:

• the meaning turned out strange
• the meaning came out strange

The verb получиться often means:

• to turn out
• to come out
• to end up being

So the idea is not just that the meaning was strange, but that because of the typo, the meaning ended up sounding strange.

Because получился agrees with смысл, which is masculine singular.

• смысл is masculine
• therefore the past tense form is получился

Compare:

• смысл получился = masculine
• фраза получилась = feminine
• предложение получилось = neuter

So the gender here is determined by смысл, not by опечатка.

This is a very common question. After verbs like получиться, оказаться, стать, Russian often uses the instrumental case for the resulting state or quality.

So:

• странный = nominative masculine singular
• странным = instrumental masculine singular

In смысл получился странным, the instrumental emphasizes the idea that the meaning turned out to be strange.

A rough pattern is:

• Он стал врачом = He became a doctor
• Она оказалась правой / права depending on structure
• Смысл получился странным = The meaning turned out strange

You may sometimes hear nominative adjectives in similar sentences in informal speech, but странным is the standard and expected form here.

Using получился adds the idea of a result.

Compare:

• смысл был странным = the meaning was strange
• смысл получился странным = the meaning came out strange / ended up strange

The second version fits this sentence better because the typo caused the strange meaning. It suggests an outcome, not just a description.

Because но means but, and it joins two clauses:

1. В письме была одна опечатка
2. но из-за неё смысл получился странным

In Russian, as in English, a comma is normally used before но when it connects two independent clauses.

It can do both, depending on context.

Here одна опечатка naturally suggests:

• one typo
• and very often, by implication, just one typo

That matters because the sentence contrasts a small cause with a noticeable effect: there was only one typo, but it made the meaning strange.

So even though одна literally means one, in this sentence it also helps create the sense of just a single typo.