Learn Hardcore Finnish: Olen hiljattain huomannut, että pilvinen sää väsyttää minua nopeasti. - I have recently noticed that cloudy weather tires me quickly.

Olen huomannut ≈ I have noticed / I’ve noticed (recently / and it’s relevant now).
Huomasin ≈ I noticed (at some point in the past, story-like, closed event).

In this sentence, olen huomannut fits well because:

the noticing happened recently, and
its result is still relevant now (you’re talking about an ongoing tendency you’ve discovered).

If you said Huomasin, että pilvinen sää väsyttää minua nopeasti, it would sound more like you’re referring to a specific past moment of noticing, not an ongoing observation.

What exactly does hiljattain mean, and are there alternatives?

Hiljattain means recently / not long ago. It’s neutral and quite common.

Close synonyms:

äskettäin – also recently, very similar in meaning.
hiljan – a bit more literary/old-fashioned.
viime aikoina – lately, in recent times (slightly more “over a period” than a single moment).

You could say, for example:

Olen äskettäin huomannut, että…
Olen viime aikoina huomannut, että…

All are correct; the nuance is very small here.

Why is hiljattain in the middle (Olen hiljattain huomannut) and can it move?

Yes, it can move. Common options:

Olen hiljattain huomannut, että… (neutral, very typical)
Hiljattain olen huomannut, että… (emphasises recently)

Putting it at the very end of the sentence, e.g.
Olen huomannut, että pilvinen sää väsyttää minua nopeasti hiljattain,
would sound odd or wrong. Time adverbs like hiljattain normally go near the verb they modify, here olen huomannut.

So the natural positions are:

just after the auxiliary (Olen hiljattain huomannut) or
at the beginning (Hiljattain olen huomannut).

Why is there a comma before että, and what does että do here?

Että is a subordinating conjunction meaning that (introducing a content clause).

Main clause: Olen hiljattain huomannut
Subordinate clause: että pilvinen sää väsyttää minua nopeasti

In Finnish punctuation, you must put a comma before että when it starts a subordinate clause like this. So:

Olen hiljattain huomannut, että … ✅
Olen hiljattain huomannut että … ❌ (considered a mistake in standard writing)

Functionally, että here is exactly like English that in “I’ve recently noticed that cloudy weather tires me quickly.”

In English I can drop that (e.g. I’ve noticed cloudy weather tires me). Can I ever drop että?

In this structure, no. Finnish is much stricter than English about this:

Olen hiljattain huomannut, että pilvinen sää väsyttää minua nopeasti. ✅
Olen hiljattain huomannut, pilvinen sää väsyttää minua nopeasti. ❌

You could rephrase the whole sentence to avoid että, for example:

Olen hiljattain huomannut pilvisen sään väsyttävän minua nopeasti. (using the VA-participle construction; more advanced and a bit formal)

But within the original structure, että cannot simply be left out.

What is the structure of pilvinen sää, and could I just say pilvinen?

Pilvinen sää = cloudy weather.

pilvinen – adjective: cloudy
sää – noun: weather

In Finnish, adjectives normally agree with the noun in case and number. Here both are nominative singular: pilvinen sää.

You can sometimes omit sää and just say pilvinen, especially if it’s clear you are talking about the weather, e.g.:

Ulkona on pilvistä / pilvinen. – It’s cloudy outside.

In your sentence, keeping sää:

makes it very clear you mean cloudy weather,
sounds perfectly natural and neutral.

You could also say:

pilvinen ilma – literally cloudy air, often used like weather
pilvinen keli – more colloquial (keli = road/weather conditions).

Why is it pilvinen sää (nominative) and not some other case like partitive?

Here pilvinen sää is the subject of the verb väsyttää:

pilvinen sää (subject)
väsyttää (verb)
minua (object/experiencer)

Subjects take the nominative in a basic sentence, so pilvinen sää stays in nominative.

If it were an object instead of a subject, then the case could change. But here, grammatically, the cloudy weather is doing the tiring, so it’s the subject.

What’s the difference between väsyttää and saying olen väsynyt?

They express related ideas, but with different structures:

Minua väsyttää. – literally It tires me / I feel tired
- väsyttää is a verb meaning to tire, to make (someone) tired.
- The cause is the subject (e.g. pilvinen sää), and the person who feels tired is in partitive (e.g. minua).
Olen väsynyt. – I am tired
- väsynyt is an adjective (“tired”),
- minä is the subject in nominative.

Your sentence:

pilvinen sää väsyttää minua nopeasti
= cloudy weather makes me tired quickly / cloudy weather tires me quickly.

If you wanted olen väsynyt style, you’d have to change the whole structure, for example:

Pilvisellä säällä olen nopeasti väsynyt. – In cloudy weather I quickly get tired.
Pilvinen sää tekee minut nopeasti väsyneeksi. – Cloudy weather makes me quickly tired.

Why is it minua and not minä, minut, or minulle?

Minua is the partitive form of minä (I).

With feeling/experience verbs like:

väsyttää (to tire),
pelottaa (to scare),
ärsyttää (to annoy),
huolestuttaa (to worry),

the experiencer (the person who feels the emotion) is normally in the partitive:

Minua väsyttää. – I feel tired / I’m being tired.
Häntä pelottaa. – He/She is scared.
Meitä ärsyttää. – We are annoyed.

So:

pilvinen sää – subject, nominative
väsyttää – verb
minua – object/experiencer, partitive

Minä väsyttää and minut väsyttää are ungrammatical.
Minulle would suggest a different verb pattern (e.g. Minulle tulee väsy etc.), not this one.

Could I say Minua väsyttää pilvinen sää nopeasti or Pilvinen sää väsyttää nopeasti minua? How flexible is the word order?

Finnish word order is quite flexible, but not all options sound equally natural.

Your original:

Pilvinen sää väsyttää minua nopeasti.
- Neutral focus on cloudy weather as the thing that tires you.

Other natural variants:

Minua väsyttää pilvinen sää nopeasti.
- Emphasises me as the experiencer: As for me, cloudy weather tires me quickly.
Pilvinen sää väsyttää nopeasti minua.
- Possible, but nopeasti usually feels better closer to the verb or at the end, and minua usually comes right after the verb. This version is not wrong, but it’s less typical.

Very odd or wrong:

Pilvinen sää minua väsyttää nopeasti. – possible in spoken Finnish as a strong emphasis fronting pilvinen sää, but marked.
Pilvinen sää nopeasti väsyttää minua. – unusual; adverb nopeasti feels misplaced.

Safest neutral options:

Pilvinen sää väsyttää minua nopeasti.
(for emphasis on you:) Minua väsyttää pilvinen sää nopeasti.

How is the adverb nopeasti formed, and could I use something else?

Nopeasti is the adverb meaning quickly, from the adjective nopea (fast).

Typical formation:

adjective base: nopea
adverb: nopeasti (add -sti)

Many Finnish adverbs of manner are formed this way:

hidas → hitaasti – slowly
selvä → selvästi – clearly
helppo → helposti – easily

Alternatives in this context:

hyvin nopeasti – very quickly
aika nopeasti – pretty quickly / fairly quickly

The position in your sentence (at the end) is very natural:

…väsyttää minua nopeasti.

Does Finnish have articles like a/the in pilvinen sää, or how do I know if it’s “the cloudy weather” or “a cloudy weather”?

Finnish has no articles (no direct equivalents of a/an/the).

Pilvinen sää can mean:

cloudy weather
the cloudy weather
a cloudy day’s weather (depending on context)

The definiteness is understood from:

context,
what has been mentioned earlier,
shared knowledge between speakers.

In English translation, you choose a or the according to the context, not because Finnish marks a difference here.

Why do we repeat the “I” information twice (in olen and in minua)? Could we leave minua out because olen already tells us it’s “I”?

The “I” in olen and the “me” in minua belong to different clauses:

Main clause: Olen hiljattain huomannut – I have recently noticed
- subject: (minä) → shown in olen
Subordinate clause: että pilvinen sää väsyttää minua nopeasti – that cloudy weather tires me quickly
- experiencer: minua

So minua is grammatically needed in the second clause; the subject of noticing (I) is not automatically the experiencer of tiring in Finnish grammar.

If you drop minua:

…että pilvinen sää väsyttää nopeasti
it becomes general/unspecified: that cloudy weather tires people/one quickly (not clearly “me”).

So if you mean specifically you, you must keep minua.

Can I say Olen huomannut hiljattain, että… instead of Olen hiljattain huomannut, että…? Is there a difference?

Yes, you can say:

Olen hiljattain huomannut, että…
Olen huomannut hiljattain, että…

Both are grammatically correct and close in meaning.

Subtle nuances:

Olen hiljattain huomannut, että… – the “recently” slightly ties more to the noticing as an event.
Olen huomannut hiljattain, että… – focuses a bit more on when this realization has occurred, but practically the same.

In everyday speech and writing, both are fine; there is no strong difference. The first version (your sentence) is more common-sounding.