После точки обычно нужна заглавная буква, а после запятой — строчная.

Because после requires the genitive case, and these two nouns decline differently.

• точка is a regular feminine noun:

  • nominative: точка
  • genitive: точки

• запятая declines like an adjective used as a noun:

  • nominative: запятая
  • genitive: запятой

So after после, you get:

• после точки
• после запятой

This is something learners notice a lot: two feminine words can behave differently if they belong to different declension patterns.

Because нужна agrees with буква, which is a feminine singular noun.

• буква = feminine singular
• therefore: нужна буква

Compare:

• нужен учебник = a textbook is needed
• нужна буква = a letter is needed
• нужно слово = a word is needed
• нужны правила = rules are needed

So нужна matches the gender and number of буква.

For the same reason: they describe буква, which is feminine.

• заглавная буква = capital letter
• строчная буква = lowercase letter

In the second half of the sentence, буква is omitted because it is already understood:

• а после запятой — строчная = а после запятой нужна строчная буква

So строчная is still feminine because the hidden noun is буква.

The dash shows that some words are being left out because they are easy to recover from the first half.

The full version would be something like:

• После точки обычно нужна заглавная буква, а после запятой обычно нужна строчная буква.

To avoid repetition, Russian often omits repeated words and uses a dash:

• а после запятой — строчная

Here the dash roughly means and after a comma, [a lowercase letter is needed].

This kind of ellipsis is very common in Russian writing.

Because а often links two things by contrast or comparison, not just simple addition.

Here the sentence compares two situations:

• after a period → capital letter
• after a comma → lowercase letter

So а is very natural because it means something like:

• whereas
• but
• while

Using и would sound more like simple addition, and the contrast would be weaker.

Обычно means usually, and it softens the statement.

Without it:

• После точки нужна заглавная буква. = After a period, a capital letter is needed.

With обычно:

• После точки обычно нужна заглавная буква. = After a period, a capital letter is usually needed.

This leaves room for exceptions, such as abbreviations, stylistic choices, or unusual formatting. Russian often uses обычно in explanations of rules to avoid sounding too absolute.

You might hear большая буква in informal speech, especially with children, but the standard grammatical term is заглавная буква.

Likewise, the standard opposite is:

• строчная буква

So if you are talking about spelling, grammar, typography, or formal writing, use:

• заглавная буква
• строчная буква

Russian word order is flexible, and both are possible.

The version in your sentence starts with После точки, which puts the focus on the condition/context first:

• After a period, usually a capital letter is needed.

That makes sense in a rule or instruction, because it tells you the situation first.

A different order is also grammatical:

• Заглавная буква обычно нужна после точки.

This version emphasizes заглавная буква a bit more.

So the original order is natural because it sounds like an explanation of punctuation rules.

Not exactly. Нужна is the short form of the adjective нужный (needed, necessary), but in sentences like this it works very much like the main predicate.

• нужный = full adjective
• нужна = short-form predicate, feminine singular

So:

• нужна заглавная буква literally means something like
• a capital letter is necessary/needed

This is a very common Russian pattern.

Compare:

• Мне нужна помощь. = I need help.
• Здесь нужна осторожность. = Caution is needed here.

Russian often uses the singular when speaking about a thing as a category or type.

So:

• нужна заглавная буква does not mean only one specific letter in one exact situation; it means the letter should be capitalized / a capital letter is required.

This is normal in both Russian and English:

• A capital letter is needed can also express a general rule.

So singular буква is completely natural here.

Yes, absolutely. That is the full, explicit version.

The sentence in your example shortens it to avoid repetition:

• а после запятой — строчная

Both are correct, but the shortened version is more elegant and natural in explanatory writing.

So you can think of the sentence as:

• После точки обычно нужна заглавная буква, а после запятой нужна строчная буква.

with the second нужна ... буква omitted because it is obvious from context.