Learn Hardcore Icelandic: Reynslan sem hún safnar núna hjálpar henni í framtíðinni. - The experience that she gathers now helps her in the future.

So -n here is the definite article (“the”) for a feminine noun in the nominative singular.
In this sentence, Reynslan is the subject of hjálpar (helps), so it appears in nominative case and takes the nominative definite ending -n.

Why is it hjálpar henni and not hjálpar hún or hjálpar hana?

This is about case and the way the verb hjálpa works.

hjálpa means to help and it governs the dative case in Icelandic. You always “help someone-DATIVE”.
The feminine third-person singular pronoun is:
- hún = nominative (she)
- hana = accusative (her as a direct object)
- henni = dative (to her / for her)

Since hjálpa needs the dative, you must say hjálpar henni (helps her in the grammatical sense “helps to-her”).
So:

Reynslan … hjálpar henni = The experience … helps her
(literally: The experience … helps to-her)

Why is hjálpar in the present tense when the meaning is about the future (“will help her in the future”)?

Icelandic often uses the present tense where English uses the future.

There are two common ways to talk about the future in Icelandic:

Present tense + a time expression
- hjálpar henni í framtíðinni
  Literally: helps her in the future
  Interpreted as English: will help her in the future
munu + infinitive (a more explicit future)
- Reynslan sem hún safnar núna mun hjálpa henni í framtíðinni.
  = The experience she is gaining now will help her in the future.

Both are correct. The original sentence uses the simple present because í framtíðinni (in the future) already makes the time reference clear.

What exactly does sem do here? Is it like “that” or “which” in English?

Yes. sem is a relative particle (often called a relative pronoun) and works much like English “that” / “which / who” in relative clauses.

Reynslan = the experience
sem hún safnar núna = that she is gathering now

Together:
Reynslan sem hún safnar núna = the experience (that) she is gaining now

Key points about sem:

It introduces a relative clause (a clause that describes a noun).
It is indeclinable (it doesn’t change form for case or gender).
Its grammatical role (subject, object, etc.) is determined by its position in the clause and the verb that follows.

Here, sem stands for the object of safnar (gathers, collects), even though sem itself doesn’t change form.

If sem stands for the object of safnar, why don’t we see any case ending on it?

Because sem itself never shows case. It has one form only, regardless of:

case (nominative / accusative / dative / genitive),
gender,
number.

The case is still there in the structure, but it’s invisible on the word sem.

In this clause:

Verb: safnar (collects, gathers, accumulates)
Verb pattern: að safna e-u = to collect / gather something (that “something” is in the dative).

In meaning, sem represents “the experience” that is being collected, so it is understood in the dative role required by safnar. But sem itself doesn’t change; it just stays sem.

Why is safnar used here? Does it really mean “is gaining experience”?

Yes. The verb að safna literally means to collect, to gather, but it is commonly used in more abstract senses like:

að safna peningum = to save / collect money
að safna frímerkjum = to collect stamps
að safna reynslu = to gain / accumulate experience

So hún safnar núna in this context means she is (now) collecting / gaining / accumulating (experience).

English often uses a more idiomatic phrase “gain experience”, but in Icelandic it’s normal to say safna reynslu (gather / collect experience).

Could we also say Reynslan sem hún er að safna núna instead of sem hún safnar núna?

Yes, that is possible, and it’s often used.

hún safnar núna
= she gathers / is gathering now (simple present, can cover progressive meaning)
hún er að safna núna
= she is in the process of gathering now (a more explicit “progressive” construction)

So:

Reynslan sem hún safnar núna…
Reynslan sem hún er að safna núna…

Both are grammatical and natural. The er að safna version puts a bit more focus on the ongoing process right now, similar to English “is gaining”. The difference is subtle; both work well here.

What is the base form of framtíðinni, and why does it end in -inni?

The base (dictionary) form is framtíð = future (a feminine noun).

Its definite forms in the singular are roughly:

Nominative: framtíðin = the future (as subject)
Accusative: framtíðina
Dative: framtíðinni
Genitive: framtíðarinnar

In the sentence we have:

í framtíðinni = in the future

Here’s what is going on:

í can take accusative or dative:
- í + accusative = motion into (into, to)
- í + dative = location / time in (in, inside, during)
We’re talking about a time period “in the future”, so we use dative.
Dative singular definite feminine of framtíð is framtíðinni.

So í framtíðinni literally means in the-the-future → in the future.

What is the difference between núna and nú? Could we say hún safnar nú reynslu?

Both nú and núna can mean “now”, but there are some nuances:

núna is very common in everyday speech for “right now / at the moment”.
nú is a bit shorter and can:
- also mean now,
- sometimes have more discourse or contrast functions (like now, look…, well then…),
- sound a bit more formal or literary in some contexts.

You can say hún safnar nú reynslu, and it is correct. It might, depending on context, sound a little more written or stylistic.
In everyday modern speech, hún safnar núna reynslu or hún er að safna reynslu núna would be more typical.

Can we change the word order and say Reynslan hjálpar henni í framtíðinni sem hún safnar núna?

No, that word order would be unnatural and confusing.

In Icelandic, the relative clause introduced by sem normally goes directly after the noun it describes:

Correct:
- Reynslan sem hún safnar núna hjálpar henni í framtíðinni.
  The experience that she is gaining now will help her in the future.

If you move sem hún safnar núna to the end:

Reynslan hjálpar henni í framtíðinni sem hún safnar núna.

then sem hún safnar núna appears to modify framtíðinni (the future that she is gaining now), which doesn’t make sense.

So keep the structure:

[Reynslan] [sem hún safnar núna] [hjálpar henni í framtíðinni].

Why is Reynslan in the nominative case? Could it be in another case here?

Reynslan is the subject of the main verb hjálpar, so it must be in the nominative.

Basic pattern:

Who/what helps her? → Reynslan (the experience).
Subject of a finite verb in Icelandic is normally in nominative case.

You would only see reynslu- forms (other cases) if the word had a different role, for example:

Hún safnar reynslu núna.
She is gaining experience now.
Here reynslu is the object of safnar, so it’s in a non-nominative case.

But in Reynslan sem hún safnar núna hjálpar henni í framtíðinni, Reynslan is the thing that does the helping, so nominative is required.