Malgraŭ la peza tornistro ŝi atingis la pinton.

Because la pinton is the direct object of atingis.

The verb atingi means to reach, and it takes a direct object: you reach something. In this sentence, the thing reached is the summit, so pinto becomes pinton.

• atingis = reached
• la pinto = the summit
• la pinton = the summit as a direct object

So:

• Ŝi atingis la pinton. = She reached the summit.

Because that phrase comes after the preposition malgraŭ.

In Esperanto, nouns after a preposition normally do not take the accusative -n. So:

• malgraŭ la peza tornistro = despite the heavy backpack

Also, the adjective must agree with the noun:

• peza matches tornistro
• both are singular and not accusative

If tornistro were a direct object, then both would take -n:

• ŝi portis la pezan tornistron = she carried the heavy backpack

But here it is part of a prepositional phrase, so no -n.

Malgraŭ is a preposition meaning despite or in spite of.

So:

• malgraŭ la peza tornistro = despite the heavy backpack

It introduces a contrast: the backpack made things harder, but she still reached the summit.

It is a very useful word for showing that something happened even though there was an obstacle.

Atingis is in the past tense.

In Esperanto:

• -as = present
• -is = past
• -os = future

So:

• atingas = reaches / is reaching
• atingis = reached
• atingos = will reach

The stem is ating-, and the ending -is shows that the action happened in the past.

Because ŝi is the subject of the verb.

In Esperanto:

• ŝi = she
• ŝin = her

The subject does the action, and the direct object receives it.

Here:

• ŝi = the person doing the reaching
• la pinton = the thing being reached

So ŝi is correct.

Yes. Esperanto word order is fairly flexible.

This sentence begins with the prepositional phrase:

• Malgraŭ la peza tornistro ŝi atingis la pinton.

But you could also say:

• Ŝi atingis la pinton malgraŭ la peza tornistro.

Both are grammatical. The first version puts more emphasis on the obstacle at the beginning: despite the heavy backpack...

Because la pinton has the accusative -n, its role is clear even if the order changes.

La is the definite article, meaning the.

So:

• la peza tornistro = the heavy backpack
• la pinton = the summit

Esperanto uses la when something is definite or identifiable. In this sentence, both the backpack and the summit are presented as specific things, not just any backpack or any summit.

Unlike English, Esperanto has only one definite article: la. It does not change for gender, number, or case.

Tornistro means a backpack, rucksack, or sometimes a satchel, depending on context.

In this sentence, heavy backpack is a very natural translation. The word suggests a bag carried on the back, especially one used for travel, hiking, or school.

So:

• peza tornistro = heavy backpack / heavy rucksack

Pinto literally means point, tip, or peak.

In mountain-climbing or hiking context, la pinto means the summit or the peak.

So in this sentence it refers to the top of the mountain or hill that she reached.

In Esperanto, adjectives agree with the nouns they describe in number and case.

Here:

• peza = heavy
• tornistro = backpack

Both are:

• singular
• not accusative

So both stay in the basic form:

• la peza tornistro

If the noun were plural, you would get:

• la pezaj tornistroj = the heavy backpacks

If it were accusative singular, you would get:

• la pezan tornistron

That agreement is one of the core grammar patterns in Esperanto.

A simple approximation is:

• mal-GRAUW

The last part, aŭ, is a diphthong, roughly like the ow in cow.

So malgraŭ sounds approximately like:

• mal-GROW or mal-GRAOW, depending on accent

A few pronunciation notes:

• g is always hard, as in go
• aŭ is one syllable-like vowel combination
• stress in Esperanto falls on the second-to-last syllable, so here the stress is on graŭ

Yes, atingi is normally used transitively: you reach something.

Examples:

• Ŝi atingis la vilaĝon. = She reached the village.
• Ili atingis sukceson. = They achieved success.
• Ni atingis la finon. = We reached the end.

That is why the thing reached takes -n.

So in your sentence:

• ŝi atingis la pinton

is exactly the expected structure.

No. Malgraŭ clearly means the backpack was an obstacle, not a help.

The idea is:

• the backpack was heavy
• even so, she reached the summit

So the sentence expresses effort or determination. If you wanted to say the backpack somehow helped, you would need a different structure and a different preposition or wording.