Hiljainen musiikki auttaa minua nukahtamaan illalla.

Because hiljainen musiikki is the subject of the sentence, and subjects are typically in the nominative case: musiikki (not musiikkia). The adjective hiljainen matches the noun in case and number, so it’s also nominative singular.

You could see hiljaista musiikkia in other sentences where it’s not the subject, e.g. Kuuntelen hiljaista musiikkia = I’m listening to quiet music (object often in the partitive).

Auttaa is the present tense, 3rd person singular form of auttaa = to help. It matches the subject hiljainen musiikki (singular): Quiet music helps…

If the subject were plural, you’d get: Hiljaiset äänet auttavat… (Quiet sounds help…).

Because auttaa commonly takes its “person being helped” in the partitive:

• auttaa minua = help me
• auttaa sinua = help you
• auttaa häntä = help him/her

So minua is the partitive of minä (I/me). In everyday Finnish this is the normal pattern with auttaa.

In standard Finnish, with the meaning help someone (in general / with something), you normally use the partitive: auttaa minua.

Minut (accusative) can appear in some constructions, but it’s not the default choice for this meaning and would usually sound different or be tied to a more specific/result-like framing. For a learner, it’s safest to learn auttaa + partitive (auttaa minua).

Nukahtamaan is nukahtaa (to fall asleep) in the 3rd infinitive illative form. This structure is very common after verbs like auttaa, mennä, tulla, etc., and it often corresponds to English to (do something) or into doing something.

So:
auttaa minua nukahtamaan = helps me (to) fall asleep.

Finnish doesn’t typically use the plain dictionary infinitive (nukahtaa) after auttaa in this meaning. Instead, it uses the pattern:

auttaa + (person in partitive) + 3rd infinitive illative
→ auttaa minua nukahtamaan

That illative ending -maan/-mään is the key piece in this construction.

Illalla is the adessive case of ilta (evening):

• ilta → illalla

The adessive is used in many time expressions to mean at / in (a time period):

• aamulla = in the morning
• päivällä = during the day
• illalla = in the evening
• yöllä = at night

Word order is fairly flexible. Illalla can move depending on emphasis:

• Hiljainen musiikki auttaa minua nukahtamaan illalla. (neutral)
• Illalla hiljainen musiikki auttaa minua nukahtamaan. (emphasizes in the evening)
• Hiljainen musiikki auttaa illalla minua nukahtamaan. (possible, a bit more marked)

The grammar stays the same; the placement mostly changes what feels highlighted.

In Finnish, double letters are long (held longer):

• musiikki has kk → the k is longer than in musiiki would be.
• nukahtamaan has aa → the a is long in that part.

Also, stress is usually on the first syllable:

• HIL-jai-nen
• MU-siik-ki
• NU-kah-taa-maan