Það sem hún sagði um málfræðina hjálpar mér mikið.

Það sem is a very common Icelandic pattern meaning what or that which.

So Það sem hún sagði means what she said.
Literally, you can think of it as that which she said, but in natural English it is usually just what she said.

A useful point:

• sem by itself often introduces a relative clause after a noun: maðurinn sem kom = the man who came
• það sem is used when there is no separate noun before sem: það sem hún sagði = what she said

Here, það is part of the fixed expression það sem. It does not mean a separate it in the normal sense.

In many contexts, það means it or that, but in það sem, it helps create the meaning what / that which.

So it is best to learn það sem as one unit.

The subject is the whole clause Það sem hún sagði um málfræðina.

That entire phrase acts like one noun phrase:

• What she said about the grammar helps me a lot.

The main verb of the whole sentence is hjálpar.
So:

• subject: Það sem hún sagði um málfræðina
• verb: hjálpar
• indirect object: mér
• adverb: mikið

Hún is the nominative form, because it is the subject of the subordinate clause hún sagði um málfræðina.

Inside that clause:

• hún = subject
• sagði = verb
• um málfræðina = prepositional phrase

So hún is in the normal subject form, just as English uses she rather than her in she said.

Because the verb hjálpa takes the dative case in Icelandic.

So:

• hjálpa mér = help me
• not hjálpa mig

This is something English speakers often need to memorize, because Icelandic verbs often require a specific case for their object.

A few examples:

• Það hjálpar mér. = That helps me.
• Hún hjálpar honum. = She helps him.
• Við hjálpum þeim. = We help them.

Málfræðina is the definite accusative singular form of málfræði.

Breakdown:

• málfræði = grammar
• málfræðin = the grammar (nominative)
• málfræðina = the grammar (accusative)

It is accusative here because it follows the preposition um, which normally takes the accusative when it means about.

So:

• um málfræði = about grammar
• um málfræðina = about the grammar

In Icelandic, prepositions govern case. That means a preposition decides what case the following noun must be in.

The preposition um usually takes the accusative. So after um, you expect an accusative form.

Examples:

• um bókina = about the book
• um kennarann = about the teacher
• um málfræðina = about the grammar

This is one of those patterns that learners usually have to memorize preposition by preposition.

Here mikið is an adverb meaning a lot or greatly. It modifies the verb hjálpar.

So:

• hjálpar mikið = helps a lot

Because it is being used adverbially, it stays in the neuter singular form mikið, which is very common in Icelandic.

Compare:

• Þetta hjálpar mikið. = This helps a lot.
• Hún vinnur mikið. = She works a lot.

If you wanted much as an adjective modifying a noun, then it would agree with that noun:

• mikla hjálp = a lot of help / great help

But that is not what is happening in this sentence.

A whole clause used as a subject is usually treated as singular in Icelandic.

So Það sem hún sagði um málfræðina is one idea, and the verb agrees with it in the singular:

• hjálpar

This is similar to English:

• What she said helps me.

Even though the subject contains many words, grammatically it counts as one unit.

Sagði is the past tense of segja = to say.

So the clause refers to something she said in the past:

• hún sagði = she said

The main verb hjálpar is present tense:

• hjálpar = helps

So the sentence combines:

• something she said earlier
• which is helping me now

That mixture of tenses is completely normal.

Not in normal usage here.

• sagði = past indicative, said
• segði = usually subjunctive or part of other structures, not the ordinary past statement form here

Since this is simply stating that she said something, sagði is the correct choice.

The word order is normal and natural.

Structure:

1. Það sem hún sagði um málfræðina = subject
2. hjálpar = verb
3. mér = object in dative
4. mikið = adverb

So the sentence follows a straightforward pattern:

• subject + verb + object + adverb

Icelandic word order can be flexible, but this version is the most neutral one.

Yes, in some contexts Icelandic allows movement for emphasis or style, but hún sagði um málfræðina is the most straightforward order here.

For a learner, it is best to keep the prepositional phrase close to sagði, because it clearly belongs with what she said.

So the standard reading is:

• what she said about the grammar

rather than attaching um málfræðina somewhere else.

A useful way to divide it is like this:

• Það sem hún sagði um málfræðina | hjálpar mér mikið

The first part is the subject clause.
The second part is the main predicate.

Inside the first part, you have another mini-sentence:

• hún sagði um málfræðina

So the sentence is layered:

1. main sentence: X hjálpar mér mikið
2. what X is: Það sem hún sagði um málfræðina

This kind of clause-as-subject structure is very common in Icelandic.