Learn Hardcore Polish: Latem słońce świeci mocniej niż jesienią. - In summer the sun shines more strongly than in autumn.

Lato is the basic noun “summer” in the nominative case.
Latem is the instrumental case, and in this context it functions like an adverb of time, meaning “in (the) summer / during summer”.

Polish often uses the instrumental case without a preposition to express time:

latem – in (the) summer
zimą – in (the) winter
nocą – at night

So Latem słońce… is best understood as “In summer, the sun…”.
If you said just Lato słońce świeci…, it would be ungrammatical; the bare nominative lato can’t fill that “when?” role.

Could I say W lecie słońce świeci mocniej… instead of Latem…? Is there a difference?

Yes, you can say:

W lecie słońce świeci mocniej niż jesienią.

This is also correct and natural. The difference is small and mostly stylistic:

Latem – instrumental case used adverbially, very common in speech and writing.
W lecie – preposition w
- locative lecie, literally “in the summer”, also common.

For most everyday contexts, Latem and W lecie are interchangeable.
Latem feels a bit more compact and maybe slightly more “bookish”, but both are standard.

What case is latem, and what case is jesienią?

Both latem and jesienią are in the instrumental singular:

lato (summer) → latem (instrumental)
jesień (autumn/fall) → jesienią (instrumental)

In these forms they act like adverbs, answering “when?”:

Latem – when? In (the) summer.
jesienią – when? In (the) autumn.

This is a very common pattern with time expressions in Polish: certain nouns in the instrumental case are used as adverbs.

Why is it jesienią and not just jesień?

Jesień is the nominative form (the basic dictionary form) meaning “autumn / fall”.
To say “in autumn / during autumn”, Polish normally uses one of these:

jesienią – instrumental used adverbially (like latem).
na jesieni – literally “on/in autumn”, with preposition na
- locative.

Jesień on its own cannot mean “in autumn”; it just names the season.
So in this sentence, because we need a time expression, we use jesienią:

Latem słońce świeci mocniej niż jesienią.
In summer the sun shines more strongly than in autumn.

Could I say Na jesieni słońce świeci… instead of jesienią?

Yes, you can also say:

Latem słońce świeci mocniej niż na jesieni.

Both jesienią and na jesieni are used to mean “in autumn”, but there is a nuance:

jesienią – very concise and neutral, works well for general statements.
na jesieni – also common; sometimes feels a bit more colloquial / conversational, and may more strongly suggest a particular autumn period.

In a general comparison like this sentence, jesienią is slightly more typical, but na jesieni would not be wrong.

What is the infinitive of świeci and how is the verb conjugated?

The infinitive is świecić – “to shine” / “to give off light”.

Present tense (imperfective) conjugation:

ja świecę – I shine
ty świecisz – you (sg.) shine
on/ona/ono świeci – he/she/it shines
my świecimy – we shine
wy świecicie – you (pl.) shine
oni/one świecą – they shine

In the sentence Latem słońce świeci…, świeci is 3rd person singular (it shines), matching słońce (the sun).

Why is there no word for “the” in słońce? Why isn’t it something like to słońce?

Polish has no articles (no a/an, no the).

słońce can mean “sun”, “a sun”, or “the sun” depending on context.

You only use to słońce in different situations, for example:

To słońce jest bardzo jasne. – This/that sun is very bright. (pointing, contrasting, or explaining)

In your sentence we’re talking about the sun in general, so simple słońce is correct and natural.

What gender is słońce, and does that affect the verb świeci?

Słońce is neuter in Polish.

In the present tense, verbs do not show gender, so świeci is the same for he / she / it – on/ona/ono świeci.

Gender becomes visible mainly in the past tense and in adjectives/participles:

Słońce świeciło. – The sun shone. (neuter ending -ło)
Jasne słońce świeciło. – The bright sun shone. (jasne = neuter form of “bright”)

Why is it mocniej and not something like bardziej mocno?

Mocniej is the comparative form of the adverb mocno (“strongly, intensely”).

For many short adverbs in -o, Polish forms the comparative with -ej:

szybko → szybciej – fast → faster
głośno → głośniej – loudly → louder
mocno → mocniej – strongly → more strongly

Bardziej mocno is technically understandable, but it sounds awkward and unnatural here.
Native speakers normally say mocniej, not bardziej mocno, in this context.

What is the difference between mocniej and mocniejszy?

mocniej is an adverb – tells you how something shines, speaks, etc.
- świeci mocniej – it shines more strongly.
mocniejszy is an adjective – describes a noun as “stronger / more intense”.
- mocniejsze słońce – a stronger sun / stronger sunlight.

So:

Słońce świeci mocniej. – The sun shines more strongly.
Słońce jest mocniejsze. – The sun is stronger.

Your sentence uses a verb + adverb pattern, so mocniej is correct:
świeci mocniej.

Why do we use niż here? Could we say mocniej od jesieni / jesienią?

In comparisons, Polish mainly uses two patterns:

Adjective + od + noun
- Słońce jest mocniejsze od lampy. – The sun is stronger than the lamp.
Adjective or adverb + niż + the compared element
- świeci mocniej niż jesienią. – it shines more strongly than in autumn.
- jest mocniejsze niż lampa. – it is stronger than the lamp.

In your sentence we are comparing two adverbial time expressions:

Latem (in summer)
jesienią (in autumn)

The natural way is: mocniej niż jesienią.
Mocniej od jesieni or od jesieni here sounds wrong or very unnatural; niż is the standard choice with an adverb like mocniej.

Can I change the word order? For example: Słońce latem świeci mocniej niż jesienią?

Yes, Polish word order is quite flexible. All of these are correct:

Latem słońce świeci mocniej niż jesienią.
Słońce latem świeci mocniej niż jesienią.
Słońce świeci latem mocniej niż jesienią.

The basic meaning stays the same. The differences are in emphasis / rhythm:

Starting with Latem gives a strong time frame: As for summer…
Słońce latem świeci… starts with the subject słońce, more neutral from an English perspective.

For a learner, Latem słońce świeci mocniej niż jesienią and Słońce latem świeci mocniej niż jesienią are the most typical-sounding variants.

Is świeci here like English “shines” or more like “is shining”? Which aspect is it?

Świeci is present tense, imperfective and it can correspond to both:

“shines” (general, habitual)
“is shining” (right now)

In this sentence, because we are talking about how things usually are in summer vs autumn, it has a general / habitual meaning:

Latem słońce świeci mocniej niż jesienią.
→ In summer, the sun (generally) shines more strongly than in autumn.

So aspect: imperfective, expressing a general truth, not a one-time event.