Kesällä ukkonen tulee nopeasti, ja taivaalla näkyy salamia.

The form kesällä is the adessive case of kesä (“summer”).

• kesä = summer
• kesällä = in summer / during summer

The adessive (-lla / -llä) often expresses:

• Location on a surface: pöydällä = on the table
• Time expressions: kesällä (in summer), talvella (in winter), aamulla (in the morning)

So Kesällä at the beginning of the sentence means “In summer” or “During the summer”.

Finnish simply does not have articles like “a / an / the” at all.

• ukkonen can mean “a thunderstorm”, “the thunderstorm”, or just “thunder” depending on context.
• salamia can mean “(some) lightnings / flashes of lightning” or “the lightnings”, again depending on context.

The definiteness or indefiniteness you express in English with “a/the” is handled in Finnish by context, word order, and sometimes case, not by separate words.

Ukkonen is a noun meaning roughly “thunder / a thunderstorm”.

Common related forms:

• ukkonen = “thunder, a thunderstorm” (basic form, nominative singular)
• ukkosta = partitive singular of ukkonen, often used in weather expressions like
  
  • Ulkona on ukkosta. = There is thunder outside.
• ukkosmyrsky = “thunderstorm” (more explicitly a storm with thunder)

In this sentence, ukkonen tulee nopeasti is like saying “a thunderstorm comes quickly” or “thunder comes quickly”.

Finnish often treats weather events as subjects that “come” or “go”:

• Talvi tulee. = Winter is coming.
• Syksy tulee aikaisin. = Autumn comes early.
• Ukkonen tulee nopeasti. = The thunderstorm comes quickly.

There are other options too:

• On ukkonen. = There is a thunderstorm.
• Ulkona on ukkosta. = There’s thunder outside.
• Ukkostaa. = It’s thundering. / There is thunder.

So ukkonen tulee is a natural Finnish way to picture the thunderstorm as approaching.

Nopea is an adjective = “fast, quick”.
Nopeasti is the corresponding adverb = “quickly”.

In Finnish, many adverbs are formed from adjectives by adding -sti:

• hidas → hidasti (slow → slowly)
• selvä → selvästi (clear → clearly)
• nopea → nopeasti (fast → quickly)

Because it modifies the verb tulee, we need the adverb:
ukkonen tulee nopeasti = “the thunderstorm comes quickly.”

Yes, here it is normal and correct.

Finnish uses commas between two independent clauses, even if they’re joined by ja (“and”):

• Menin kotiin, ja hän jäi töihin.
• Kesällä ukkonen tulee nopeasti, ja taivaalla näkyy salamia.

Both parts have their own subject and verb:

1. (Kesällä) ukkonen tulee nopeasti.
2. Taivaalla näkyy salamia.

Because they are full clauses, you normally use a comma before ja in standard written Finnish.

Taivaalla is again the adessive case (-lla / -llä).

• taivas = sky, heaven
• taivaalla = “in the sky / on the sky (visibly up there)”
• taivaassa = inessive case, more literally “inside the sky / in heaven”

In everyday language, for “in the sky (up above, visible)”, taivaalla is the normal choice:

• Lintuja on taivaalla. = There are birds in the sky.
• Taivaalla näkyy salamia. = Lightning flashes are visible in the sky.

Taivaassa is more likely in religious or metaphorical contexts, e.g. Taivaassa on rauha. (“There is peace in heaven.”)

Both come from the root nähdä (“to see”), but:

• näkee = “(someone) sees (something)”
  
  • Hän näkee salaman. = He/she sees a lightning flash.
• näkyy = “is visible / can be seen” (no human subject mentioned)
  
  • Taivaalla näkyy salamia. = Lightning flashes are visible in the sky.

So näkyy is intransitive: it describes how something appears or is visible, without saying who is doing the seeing.

Salama = “(a) flash of lightning”. Its plurals:

• salamat = nominative plural (“the lightning flashes” as a whole set)
• salamia = partitive plural (“(some) lightning flashes”, indefinite amount)

Partitive plural often expresses:

• An indefinite quantity
• Something not seen as a complete, counted set

Taivaalla näkyy salamia suggests some flashes of lightning, a non‑specific number.
If you said Taivaalla näkyvät salamat, it would feel more like “the lightning flashes (all of them there) are visible”, more definite and a bit heavier in style.

Yes, it’s grammatically correct but the nuance changes:

• Taivaalla näkyy salamia.

  • Neutral, everyday style.
  • Focus on the existence / presence of some lightning flashes in the sky.

• Taivaalla näkyvät salamat.

  • More definite and slightly more formal or descriptive.
  • Literally “The lightning flashes that are visible in the sky…”
  • You might follow it with more information: Taivaalla näkyvät salamat ovat hyvin kirkkaita.

The original sentence with salamia sounds more typical for general description.

Yes, but it changes the structure:

• Salamoida = “to flash lightning / to lightning” (impersonal weather verb)
• Taivaalla salamoi. = “There is lightning in the sky.” / “It’s lightning(ing) in the sky.”

Compare:

• Taivaalla näkyy salamia. = “Lightning flashes are visible in the sky.”
  (focus on what you see)
• Taivaalla salamoi. = “There’s lightning going on in the sky.”
  (focus on the weather activity)

Both are natural; the original sentence just chooses to talk about visible flashes using näkyy salamia.

Finnish has one present tense (here: tulee, näkyy) that covers both:

• English simple present (comes, is visible)
• English present continuous (is coming, is being seen)

Which English form you use depends on context, not on a different Finnish tense:

• Kesällä ukkonen tulee nopeasti.
  
  • Can be translated as:
    
    • “In summer, the thunderstorm comes quickly.”
    • “In summer, the thunderstorm comes quickly (when it comes).”
      It describes a general habit or typical situation.

Similarly näkyy = “is visible / can be seen”; there’s no separate progressive form in Finnish.

Yes, Finnish word order is flexible, and these alternatives are possible:

• Ukkonen tulee nopeasti kesällä.

  • Still correct; now ukkonen is more foregrounded, and kesällä is mentioned later as extra information.
  • The basic meaning doesn’t change, but the emphasis shifts.

• Salamia näkyy taivaalla.

  • Also correct; it emphasizes salamia (some lightning flashes) a bit more first.
  • Taivaalla näkyy salamia vs Salamia näkyy taivaalla is mostly a matter of what you present first in the sentence.

The original order with Kesällä first highlights the time frame (“In summer”) as the starting point of the statement.