Learn Hardcore Finnish: Ystäväni oli myös hermostunut, mutta yritti näyttää rennolta. - My friend was also nervous, but tried to look relaxed.

Both ystäväni and minun ystäväni are correct. Using only the suffix is very normal and not especially emphatic; adding minun usually adds some focus to whose friend it is.

Can Ystäväni mean both my friend and my friends?

Yes. Ystäväni is ambiguous between singular and plural:

ystäväni = my friend
ystäväni = my friends

The form is the same for:

singular nominative with -ni: ystävä
- -ni
plural nominative with -ni: ystävät → ystäväni (the -t drops before the possessive suffix)

Context usually makes it clear whether one or more friends are meant. In your sentence, you’d normally understand it as singular (my friend) unless the wider text suggests otherwise.

What exactly is hermostunut, and why is it used with oli?

Hermostunut is the past participle of the verb hermostua (to get nervous), but it is commonly used as an adjective meaning nervous.

hän hermostui = he/she got nervous (the change happened)
hän oli hermostunut = he/she was nervous (describing the state)

So in Ystäväni oli myös hermostunut, oli hermostunut functions like was nervous.

Grammatically, Finnish often uses olla (to be) + a participle (hermostunut) to describe a resulting state:

Olen väsynyt. = I am tired.
Hän oli hermostunut. = He/she was nervous.

Here it is not a perfect tense (like English had been nervous), just a description of a state in the past.

Where does myös go in the sentence, and could it be placed somewhere else?

In the sentence Ystäväni oli myös hermostunut, myös means also / too.

Typical positions:

Ystäväni oli myös hermostunut.
→ Neutral: My friend was also nervous.
Myös ystäväni oli hermostunut.
→ Slightly more emphasis on my friend also (as opposed to others).
Ystäväni oli hermostunut myös.
→ Possible, but often sounds a bit heavier or marked; context decides if it’s natural.

So myös is fairly flexible in position, but where you put it affects what is being emphasized:

Before the subject (Myös ystäväni) → Also my friend (not just others)
Before the predicate adjective (oli myös hermostunut) → Was also nervous (in addition to some other qualities / people)

In your sentence, the usual reading is simply that your friend, in addition to someone else, was nervous.

Why is there no explicit subject in the second clause (…mutta yritti näyttää…)?

Finnish often omits the subject when it is clear from context and stays the same as in the previous clause.

Ystäväni oli myös hermostunut, mutta yritti näyttää rennolta.
Literally: My friend was also nervous, but [he/she] tried to look relaxed.

The verb yritti is in 3rd person singular past, so the understood subject is hän (he/she), which in context refers back to ystäväni.

You could explicitly say:

…, mutta hän yritti näyttää rennolta.

That is also correct, just a bit more explicit. Omitting hän here is very natural style in Finnish.

What does yritti tell me about the tense and person?

Yritti is from the verb yrittää (to try):

infinitive: yrittää
3rd person singular past: hän yritti = he/she tried

So yritti by itself encodes:

person: 3rd person singular (he/she)
tense: past

This matches the subject ystäväni from the first clause: My friend … tried …

What does näyttää mean here? Is it to show or to look / seem?

Näyttää has two main meanings:

to show something
- Näytän sinulle kuvan. = I’ll show you a picture.
to look / seem / appear (in the sense of appearance)
- Näytät väsyneeltä. = You look tired.

In yritti näyttää rennolta, it’s the second meaning:

to try to look / appear relaxed

So the structure is:

yrittää
- infinitive: yritti näyttää = tried to look
näyttää
- adjective in ablative (see next question): näyttää rennolta = to look relaxed

Why is rennolta in that special -lta form instead of just rento?

Rennolta is the ablative singular case of rento (relaxed).

The general pattern with näyttää (to look / seem) when talking about someone’s appearance or seeming state is:

näyttää
- adjective in -lta / -ltä (ablative)
  - Näytät väsyneeltä. = You look tired.
  - Hän näyttää iloiselta. = He/she looks happy.
  - Yritti näyttää rennolta. = (He/she) tried to look relaxed.

So:

rento (basic form) → renno-lta (ablative): rennolta

The ablative here expresses “seems/looks like X”.

Why does rento change to renno- before adding -lta?

This is a stem change that some words undergo. The adjective rento has two stems:

basic form: rento
“stronger” stem: renno- (used in many case forms)

Examples:

nominative: rento
genitive: rennon
ablative: rennolta
inessive: rennos sa → written rennoissa (plural)

So the -ltA case ending is added to the strong stem renno-, giving rennolta.

From a learner’s point of view, the important points are:

dictionary form: rento = relaxed
with näyttää, use rennolta to say looks relaxed

Is the comma before mutta used like in English, and what does mutta mean?

Yes, very similarly.

mutta = but

A comma is normally used before mutta when it joins two clauses:

Ystäväni oli myös hermostunut, mutta yritti näyttää rennolta.
= My friend was also nervous, but tried to look relaxed.

So the punctuation and the meaning of mutta are roughly the same as English but in this kind of sentence.