Ystäväni harrastaa jalkapalloa viikonloppuna.

Ystäväni means my friend.

• ystävä = friend
• -ni = the possessive suffix for “my”

Finnish often shows possession with a suffix instead of (or in addition to) a separate word like minun (my). So:

• ystäväni = my friend
• ystäväsi = your friend
• ystävänsä = his/her/their friend

You can also say minun ystäväni, but usually the -ni on the noun is enough.

Finnish does not have articles like a/an or the. Context tells you whether a noun is specific or general.

So:

• ystäväni can mean my friend or my (particular) friend depending on context.
• jalkapalloa just means (some) football, no article needed.

Where English needs to choose between a/the friend or a/the weekend, Finnish simply uses ystäväni, viikonloppuna, etc., with no extra words.

Harrastaa means to have as a hobby / to engage in regularly. It’s about what you do in your free time as a pastime.

• harrastaa jalkapalloa ≈ to play football as a hobby, to do football (as a pastime)
• pelata jalkapalloa = to play football (literally play a game of football)

So:

• Ystäväni harrastaa jalkapalloa.
  → My friend does/plays football as a hobby.

If you say:

• Ystäväni pelaa jalkapalloa viikonloppuna.
  → My friend plays football on the weekend (focus on the activity, not necessarily that it’s a hobby).

In many everyday situations both are possible, but harrastaa emphasizes the hobby aspect.

Jalkapalloa is the partitive case of jalkapallo (football).

• jalkapallo = football (basic form)
• jalkapalloa = football (partitive)

With harrastaa, the thing you “do as a hobby” is almost always in the partitive:

• harrastaa musiikkia = to do music / to be into music
• harrastaa juoksua = to do running as a hobby
• harrastaa jalkapalloa = to play football as a hobby

The partitive here mainly signals an ongoing, unbounded activity, not a single, complete, countable event.

No. That sounds wrong to a native speaker.

With harrastaa, the object is normally in the partitive:

• ✅ harrastaa jalkapalloa
• ❌ harrastaa jalkapallo

So you should keep the -a: jalkapalloa.

Viikonloppuna means on the weekend / during the weekend.

• viikonloppu = weekend
• viikonloppuna = on/over/during the weekend

The -na ending is the essive case. One of its uses is in time expressions, meaning something like “at / during (a point or period in time)”:

• kesänä = in the summer
• talvena = in the winter
• syntymäpäivänä = on (the) birthday
• viikonloppuna = on/over the weekend

By itself, viikonloppuna usually means (on) the weekend in a fairly general or context-dependent way:

• It could refer to the coming weekend, the last weekend, or just weekends in general, depending on context.

If you clearly want to say “on weekends / every weekend”, Finnish often uses:

• viikonloppuisin = on weekends / at weekends (habitually)

Examples:

• Ystäväni harrastaa jalkapalloa viikonloppuna.
  → My friend plays football on the weekend. (context decides if it’s this weekend, that weekend, generally, etc.)

• Ystäväni harrastaa jalkapalloa viikonloppuisin.
  → My friend (habitually) plays football on weekends.

Harrastaa is in the present tense, 3rd person singular:

• infinitive: harrastaa = to have (something) as a hobby
• stem: harrasta-
• 3rd person singular ending: -a / -ää (depending on vowel harmony)

So:

• harrastaa = he/she/it practices / does (as a hobby)

Other forms for comparison:

• (minä) harrastan = I do as a hobby
• (sinä) harrastat = you do as a hobby
• (me) harrastamme = we do as a hobby

Yes, Finnish word order is fairly flexible, and your alternative is correct.

• Ystäväni harrastaa jalkapalloa viikonloppuna.
• Viikonloppuna ystäväni harrastaa jalkapalloa.

Both are grammatical. The difference is mostly in emphasis and flow:

• Starting with Ystäväni emphasizes who does the action.
• Starting with Viikonloppuna emphasizes when it happens.

The core meaning remains the same.

You can say it; it’s grammatically correct.

• minun = my (separate pronoun)
• ystäväni = my friend (with possessive suffix -ni)

However, in normal speech you usually don’t need both. Typically you choose one:

• Ystäväni harrastaa jalkapalloa. (most common)
• Minun ystäväni harrastaa jalkapalloa. (possible, can add emphasis: my friend, not someone else’s)

Yes, jalkapallo is one compound word:

• jalka = foot
• pallo = ball
  → jalkapallo = football

So the structure is very literal, like English “football”. The hobby form in your sentence is jalkapalloa (partitive case).

Sure:

• Ystäväni

  • ystävä = friend
  • -ni = my
    → my friend

• harrastaa
  → practices / does as a hobby

• jalkapalloa

  • jalkapallo = football
  • -a (partitive) = (ongoing / unbounded activity)
    → (some) football (as a hobby)

• viikonloppuna

  • viikonloppu = weekend
  • -na (essive) = at/during (a time)
    → on/during the weekend

Combined sense: “My friend does/plays football as a hobby on the weekend.”