Learn Hardcore Finnish: Jos olen väsynyt, liikuntatunti piristää minua nopeasti. - If I am tired, the PE class cheers me up quickly.

jos is used for hypothetical or uncertain conditions, much like English if.
Finnish also has kun, which can sometimes translate as when or whenever, and sometimes as if, but it usually suggests something more factual or expected than jos.
- Kun olen väsynyt, liikuntatunti piristää minua nopeasti.
  = When(ever) I am tired, an exercise class quickly cheers me up. (sounds more like something that actually happens regularly)
- Jos olen väsynyt, …
  = If I am (at that time) tired, … (more conditional/hypothetical)

Why is piristää in the present tense instead of a conditional form like piristäisi?

Piristää is the normal present tense, 3rd person singular form:

liikuntatunti piristää = the exercise class cheers (me) up / refreshes (me).

In Finnish, you typically use the normal present tense for general truths, habits, and typical results of conditions:

Jos olen väsynyt, liikuntatunti piristää minua nopeasti.
= If I am tired, an exercise class (typically) cheers me up quickly.

The conditional form piristäisi would sound more like:

Jos olisin väsynyt, liikuntatunti piristäisi minua nopeasti.
= If I were tired, an exercise class would cheer me up quickly.
This is a more hypothetical, “unreal” condition (like English if I were).

Why is there a comma between Jos olen väsynyt and liikuntatunti piristää minua nopeasti?

In Finnish, you always put a comma between an introductory jos-clause and the main clause that follows it.

So:

Jos olen väsynyt, liikuntatunti piristää minua nopeasti.

If the order were reversed, you would not put a comma:

Liikuntatunti piristää minua nopeasti jos olen väsynyt.
(no comma in this order)

What exactly is liikuntatunti? Why is it one long word?

Liikuntatunti is a compound noun:

liikunta = physical exercise, physical education
tunti = hour, lesson, class

Together:

liikuntatunti = exercise class / PE class / physical education lesson

In Finnish, compound nouns are normally written as one word, not separated or hyphenated like in English. So you don’t write liikunta tunti, it must be liikuntatunti.

Why is it minua and not minut after piristää?

Minua is the partitive form of minä (I, me).
Minut is the accusative form.

With many verbs of feeling, experience, or mental/physical state, Finnish tends to use the partitive object, especially with persons:

Kahvi piristää minua. = Coffee cheers me up / wakes me up.
Liikuntatunti piristää minua nopeasti. = An exercise class quickly cheers me up.

The full-affected/complete-object idea (minut) is possible in some contexts, but minua is the natural, usual choice with piristää in the sense of brightening/cheering (someone) up.

So:

minä (nominative, subject)
minua (partitive, often like “me” in certain object/experience roles)
minut (accusative, “me” as a complete object in some structures)

Here minua is standard.

Could you say minut here at all, and would it change the meaning?

You can see piristää minut in some contexts, but it feels different:

Kahvi piristi minut täysin.
= The coffee completely perked me up.

Using minut emphasizes the idea of a complete change of state, like “it got me fully back to normal”.
Using minua is more neutral and common for “cheers me up / refreshes me” in general.

In the given sentence, minua is the natural choice.
…piristää minut nopeasti is possible but sounds stronger and more result-focused (“gets me back on my feet quickly”).

Why is it olen väsynyt and not something like olen väsynyttä or väsynytnä?

Väsynyt is an adjective meaning tired.

In Finnish, when you say “I am X” (where X is an adjective), you use:

nominative form of the adjective as a predicative:
- Olen väsynyt. = I am tired.
- Olet iloinen. = You are happy.
- Hän on pitkä. = He/She is tall.

Forms like väsynyttä (partitive) or väsynytnä (essive “as tired”) are used in more specific structures, not for the basic “to be + adjective” meaning:

Olen vähän väsynyt. = I am a bit tired. (still nominative)
Olen väsynytnä töissä. = I am (in the state of being) tired at work. (very marked and unusual; not how you normally say it)

So here olen väsynyt is simply I am tired with a normal predicative adjective in the nominative.

Does this Finnish sentence describe a single situation, or a general habit, or both?

In Finnish, the present tense is often used for:

current situations
general truths
habitual actions

So:

Jos olen väsynyt, liikuntatunti piristää minua nopeasti.

Can mean:

“If I am tired (on that occasion), an exercise class cheers me up quickly.”
or “Whenever I’m tired, an exercise class quickly cheers me up.” (habit/general pattern)

Context would decide which nuance is intended, but grammatically both readings are natural.

Where can the adverb nopeasti go? Is the position fixed?

Nopeasti means quickly and is an adverb. The most neutral position is exactly as in the sentence:

Liikuntatunti piristää minua nopeasti.

Other possible orders:

Liikuntatunti piristää nopeasti minua.
(possible, but a bit less natural; emphasis can sound more on “quickly”)
Liikuntatunti nopeasti piristää minua.
(unusual, marked emphasis; not typical everyday word order)

Default, neutral word order is:

[Subject] [Verb] [Object] [Adverb]
- Liikuntatunti piristää minua nopeasti.

Can you omit minua and just say Jos olen väsynyt, liikuntatunti piristää nopeasti?

You can omit minua, but the meaning shifts slightly:

Liikuntatunti piristää minua nopeasti.
= The exercise class cheers me up quickly. (explicit who is affected)
Liikuntatunti piristää nopeasti.
= The exercise class perks (one) up quickly / is quickly refreshing. (more general)

Without minua, it feels more like a general statement about the class being refreshing, not specifically about you as the object.

In the original sentence, since we are talking about “if I am tired”, it is natural and clear to keep minua to show that I am the person being cheered up.