היא לא רוצה הרבה שמן בעוגה, כדי שהיא לא תהיה כבדה.

It is written as one word, but it is really ש־ + היא.

• ש־ means that
• היא means she / it (feminine)

So כדי שהיא לא תהיה כבדה is literally something like so that she/it will not be heavy.

Because the second part is a full clause, and Hebrew often states the subject again in that kind of clause. English might say so it won’t be heavy, and Hebrew can say כדי שהיא לא תהיה כבדה.

In this sentence, the second היא is understood from context as referring to the cake, not necessarily to the woman in the first clause.

After כדי ש־ (so that / in order that), Hebrew commonly uses the future tense to express the intended result.

So:

• תהיה = will be
• לא תהיה כבדה = will not be heavy

Even though English might sometimes use isn’t in a similar idea, Hebrew usually uses the future here.

Because it agrees with the thing being described, which is עוגה (cake), a feminine noun.

So:

• עוגה = feminine singular
• כבדה = feminine singular adjective

If the noun were masculine, the adjective would normally be masculine too.

In isolation, it is a little ambiguous, because היא can mean she or it for a feminine noun. But the context makes the cake the most natural meaning: she does not want a lot of oil in the cake, so that the cake will not be heavy.

If you wanted to make it completely explicit, you could say כדי שהעוגה לא תהיה כבדה.

In unpointed Hebrew, both masculine singular and feminine singular are often written the same in the present tense.

So רוצה can be:

• rotze = masculine
• rotza = feminine

Here, because the subject is היא, it is feminine: היא לא רוצה = she does not want.

שמן here is a mass noun, like oil in English, so Hebrew uses הרבה in the sense of a lot of / much.

So:

• הרבה שמן = a lot of oil / much oil

Also, in modern Hebrew, הרבה is very commonly used with both countable and uncountable nouns.

Here it means oil.

The word שמן can also be an adjective meaning fat, but in this sentence it is clearly a noun because of the context:

• הרבה שמן בעוגה = a lot of oil in the cake

In a baking sentence, oil is the natural interpretation.

It is the preposition ב־ (in) attached directly to עוגה (cake). Hebrew prepositions like ב־, ל־, and כ־ are often written as prefixes.

So:

• ב + עוגה → בעוגה

In unpointed Hebrew, בעוגה can represent either in a cake or in the cake; context tells you which one is meant.

Yes. That is very natural Hebrew.

• כדי שהיא לא תהיה כבדה
• כדי שלא תהיה כבדה

Both mean roughly so that it won’t be heavy. The version without היא is a little more compact, and it works when the subject is clear from context.