Learn Hardcore Russian: Мне нравится лёгкий завтрак.

Мне нравится лёгкий завтрак = A light breakfast is pleasing to me.
мне is dative (to me).
The thing that is liked (лёгкий завтрак) is the subject in the nominative.
The person who likes it (мне) is an indirect object in the dative.

So you say:

Мне нравится завтрак. – A breakfast is pleasing to me.

You do not say я нравлюсь завтрак; я нравлюсь means someone likes me (I am pleasing to someone).

What is the difference between Мне нравится лёгкий завтрак and Я люблю лёгкий завтрак?

Both can be translated I like a light breakfast, but they feel different:

Мне нравится лёгкий завтрак
- More neutral and descriptive.
- Often about impressions, tastes, current preferences.
- Similar to I like / I enjoy.
Я люблю лёгкий завтрак
- Stronger, more personal, more emotional or habitual.
- Often about stable tastes, habits, or love/affection.
- Similar to I really like / I love in English.

Both are correct; нравится simply sounds a bit softer and more objective.

Why is the verb нравится in third person singular?

The verb agrees with the thing that is liked, not with the person who likes it.

In Мне нравится лёгкий завтрак:

Subject: лёгкий завтрак (a light breakfast) – singular.
Verb: нравится – 3rd person singular form.
Indirect object: мне – to me (dative).

So you get:

Мне нравится завтрак. – The breakfast pleases me.
If the subject were plural, the verb would change:
Мне нравятся лёгкие завтраки. – I like light breakfasts.

How would the sentence change if I want to say “I like light breakfasts” (plural)?

You need both a plural noun and a plural verb:

Мне нравятся лёгкие завтраки.

Changes:

завтраки – plural of завтрак.
лёгкие – plural form of the adjective to agree with завтраки.
нравятся – 3rd person plural form of нравиться to agree with the plural subject.

Why is the adjective лёгкий in that form? Why not лёгкого or something else?

лёгкий agrees with завтрак in:

gender: masculine
number: singular
case: nominative

In this construction, лёгкий завтрак is the subject of the sentence, so it must be in the nominative case:

лёгкий завтрак – nominative masculine singular.
That is why you see лёгкий, not лёгкого (genitive/accusative) or other forms.

What exactly does лёгкий завтрак mean? Is it “easy breakfast” or “light breakfast”?

In this context, лёгкий завтрак means light breakfast – not heavy or not very filling.

лёгкий can mean several things:

not heavy (in weight) – a light bag
not heavy (in food) – a light meal
not difficult – an easy task

With food words (like завтрак, обед, ужин), лёгкий is understood as:

not greasy, not rich, not very big
easy to digest

If you wanted to say simple rather than light, you could use простой завтрак (a simple breakfast).

How do I know that завтрак here is nominative and not accusative? The forms look the same.

Masculine inanimate nouns like завтрак have the same form in nominative and accusative singular:

nominative singular: завтрак
accusative singular: завтрак

You tell the case from the grammar of the sentence, not from the ending:

With нравиться, the thing that is liked is the subject, so it must be nominative.
The person who likes it is in the dative (мне).

So in Мне нравится лёгкий завтрак, grammar tells us:

лёгкий завтрак = nominative subject
мне = dative indirect object

Can I change the word order? For example, Лёгкий завтрак мне нравится?

Yes, Russian allows relatively free word order. All of these are possible and grammatical:

Мне нравится лёгкий завтрак. – neutral, most typical.
Лёгкий завтрак мне нравится. – emphasizes лёгкий завтрак (as opposed to some other kind).
Лёгкий завтрак нравится мне. – emphasizes мне (to me in particular).
Нравится мне лёгкий завтрак. – more emotional, poetic, or stylistically marked.

The meaning stays basically the same; word order mainly affects emphasis and style.

What is the tense/aspect of нравится? How is it different from понравился?

нравится is the present tense of the imperfective verb нравиться:

Мне нравится лёгкий завтрак. – I (generally/now) like a light breakfast.

понравиться is the perfective counterpart:

Мне понравился лёгкий завтрак. – I liked the light breakfast (on that occasion; it pleased me).

So:

нравится / нравился – ongoing state, general liking, description.
понравится / понравился – a completed event, the moment you started liking something or how it turned out.

How do I pronounce Мне нравится лёгкий завтрак correctly?

Key points:

Мне – [mnye], single syllable, consonant cluster mn.
нравится – stress on the first syllable: [NRÁ-vi-tsa].
лёгкий – stress on ё: [LYÓH-kiy].
- ё is always stressed and pronounced like yo.
- The written гк is actually pronounced with хк ([хк]) in standard speech.
завтрак – stress on за́в: [ZÁV-trak].

So, approximately: [Mnye NRÁ-vi-tsa LYÓH-kiy ZÁV-trak].

Could I say something like Мне нравится легко завтракать instead?

That would sound odd or wrong in Russian.

Reasons:

легко is an adverb (lightly, easily), not the right way to describe the meal itself.
To express the general preference for a light breakfast, Russian naturally uses a noun phrase:
- Мне нравится лёгкий завтрак.

If you really want to use a verb, you might say:

Мне нравится легко завтракать – would be understood more as I like to have breakfast easily / without effort, which is strange.

So for the idea I like having a light breakfast, stick with the noun phrase Мне нравится лёгкий завтрак.

What is the difference between Мне нравится лёгкий завтрак and Мне нравится завтракать легко?

They are grammatically different and sound different:

Мне нравится лёгкий завтрак.
- Natural, idiomatic.
- Focus on what kind of breakfast you like: a light one.
- Uses noun завтрак.
Мне нравится завтракать легко.
- Grammatically possible but unusual.
- Sounds like I like to breakfast lightly/easily – a bit abstract or stylistically odd.
- Uses verb завтракать (to have breakfast) plus adverb легко.

For everyday speech about food preferences, Мне нравится лёгкий завтрак is the normal way to say it.