Новая грамматическая тема: конструкция «вот бы», которая выражает сильное желание.

Russian often leaves out есть (is/are) in the present tense, especially in headings, labels, and short definitions.

So Новая грамматическая тема: is basically:

• A new grammar topic:
• literally, something like [This is a] new grammar topic:

This sounds completely natural in Russian.

Because they both describe тема, and тема is a feminine noun.

In Russian, adjectives must agree with the noun they describe in:

• gender
• number
• case

So:

• новая тема = new topic
• грамматическая тема = grammar topic / grammatical topic

Both adjectives take feminine singular nominative endings because тема is feminine singular nominative.

Here конструкция вот бы is part of a heading-like structure after the colon. It is naming the topic, so конструкция stays in the nominative case.

You can think of it as:

• The new grammar topic: the construction вот бы

Since it is not acting as a direct object or following a preposition, there is no reason to change it to another case here.

Вот бы is a fixed expression used to show a strong wish, often about something the speaker would really like to happen.

Common English equivalents are:

• if only ...
• I wish ...
• would that ... (more literary)

For example:

• Вот бы сейчас домой! = If only I were home now!
• Вот бы поехать к морю. = I wish I could go to the sea.

It usually sounds emotional, dreamy, or strongly wishful.

Бы by itself is a particle with several uses, especially in conditional or hypothetical meanings. It often needs other words around it.

Вот бы is more of a set expression. The word вот adds emotional force, so the whole construction sounds like a vivid wish:

• Я хотел бы... = I would like...
  This is more neutral and polite.
• Вот бы... = If only... / I really wish...
  This is more emotional and expressive.

So вот бы is not just random вот + бы in this kind of sentence; it is a recognized wish-construction.

Которая refers to конструкция.

Since конструкция is:

• feminine
• singular
• nominative

the relative pronoun also appears as feminine singular nominative:

• конструкция, которая выражает...
• the construction that expresses...

This is exactly like agreement in English the construction that..., except Russian shows the connection more clearly through the form of the pronoun.

Because которая выражает сильное желание is a relative clause.

In Russian, relative clauses introduced by words like:

• который
• которая
• которое
• которые

are normally set off with a comma.

So:

• конструкция, которая выражает сильное желание

This is standard Russian punctuation.

Russian commonly uses the present tense to state general facts, definitions, and descriptions.

So которая выражает сильное желание means:

• which expresses a strong desire

This is not about something happening right now in time. It is a general statement about what the construction does.

English does the same thing:

• A noun is a word that names a person, place, or thing.
• A relative clause is a clause that modifies a noun.

Because желание is a neuter noun, and here it is the direct object of выражает (expresses).

So:

• dictionary form: сильное желание
• meaning: strong desire

This happens to look the same as nominative singular because many neuter nouns and adjectives have the same form in nominative and accusative when inanimate.

Breakdown:

• желание = neuter singular noun
• сильное = adjective agreeing with желание

A few very common patterns are:

1. вот бы + infinitive

• Вот бы поехать в Петербург.
  If only I could go to St. Petersburg.

• Вот бы отдохнуть.
  I wish I could get some rest.

2. вот бы + past-tense form
This can describe a wished-for situation.

• Вот бы он пришёл.
  If only he came / I wish he would come.

• Вот бы было тепло.
  If only it were warm.

3. вот бы + noun / short exclamation

• Вот бы лето!
  If only it were summer!

• Вот бы выходной!
  If only it were a day off!

In all of these, the feeling is stronger and more emotional than a plain statement with хотеть.

It is very natural in everyday speech and writing, but it is usually emotional and conversational rather than formal.

You can absolutely see it in books, dialogue, and ordinary speech. But in a formal academic or business context, people would usually choose more neutral wording.

Compare:

• Я хотел бы задать вопрос. = I would like to ask a question.
  Polite, neutral, formal enough.

• Вот бы задать ему вопрос... = If only I could ask him a question...
  More emotional, personal, and expressive.

Usually it expresses something the speaker wants but does not currently have, or something uncertain, difficult, or hypothetical.

That is why it often feels like:

• if only
• I wish
• would be nice if

It does not normally present a plain factual desire the way я хочу does.

Compare:

• Я хочу спать. = I want to sleep.
  Simple statement of desire.

• Вот бы поспать. = If only I could sleep / I wish I could get some sleep.
  More wistful, emotional, and often suggests that sleeping is not easy right now.