La guía nos ayudó a situar el castillo en el mapa y luego mi hija intentó situar el río ella sola.

Because different verbs combine with the infinitive in different ways.

• ayudar a + infinitive = to help to do something
  
  • nos ayudó a situar el castillo
• intentar + infinitive = to try to do something
  
  • mi hija intentó situar el río

So:

• ayudó a situar is correct
• intentó situar is correct
• intentó a situar is not correct

This is something you often just have to learn verb by verb.

Nos means us.

In La guía nos ayudó..., it tells you who received the help: the guide helped us.

A useful grammar point here is that with ayudar, the person being helped is treated as a direct object in standard Spanish:

• La guía ayudó a los turistas
• La guía nos ayudó

Even though a full noun would often appear with the personal a (a los turistas), the pronoun is still the direct-object form:

• me, te, lo/la, nos, os, los/las

So nos here is not to us in the indirect-object sense. It is simply us.

Here situar means something like:

• to locate
• to place correctly on a map
• to identify where something is

So in this context, it does not mean physically moving the castle or river. It means finding or marking their location.

This verb is quite natural in geography, explanations, maps, and school-type tasks.

Because the sentence describes two separate but parallel actions:

• first, the guide helped us locate the castle
• then, my daughter tried to locate the river by herself

Repeating situar is normal Spanish. It keeps the structure clear and shows that both actions are the same kind of task.

You could avoid repetition in some contexts, but here it sounds natural.

Yes. Depending on context, you might also hear:

• localizar = to locate
• ubicar = to locate, place

But there is a regional nuance:

• In Spain, situar and localizar are very natural here.
• Ubicar is also understood, but it is often felt to be more common in much of Latin America.

Colocar would usually suggest physically putting something somewhere, so it would be less natural for identifying a place on a map.

Because the sentence tells a sequence of completed actions in the past:

• the guide helped us
• then my daughter tried

That is exactly the kind of situation where Spanish commonly uses the preterite:

• ayudó
• intentó

If you used the imperfect instead, it would suggest background, habit, or an ongoing situation rather than two finished events.

Because the guide is female.

For a person, guía can refer to either a male or a female guide, and the article shows the gender:

• el guía = the male guide
• la guía = the female guide

So La guía nos ayudó... tells you that the guide was a woman.

Spanish uses definite articles very often before nouns, including when talking about specific things already understood in context.

Here, el castillo and el río sound natural because they refer to specific items the speakers are trying to locate.

So:

• situar el castillo
• situar el río

is the normal phrasing.

English and Spanish do not always use articles in exactly the same way, so this is something learners notice a lot.

Because en is the natural preposition for location within the map as a represented space:

• situar el castillo en el mapa

This means locating it on the map in the English sense.

It is worth comparing:

• en el mapa = on the map, as part of the map representation
• sobre el mapa = physically on top of the map

So if you are talking about finding where something belongs geographically, en el mapa is the normal choice.

Because Spanish usually leaves subject pronouns out when they are not needed.

Here the subject is already clearly stated:

• mi hija intentó...

So there is no need to add ella before the verb.

Spanish would only add the subject pronoun if there were a reason, such as:

• emphasis
• contrast
• clarification

That is why ella appears later in ella sola, where it adds emphasis.

Ella sola means:

• by herself
• on her own
• she herself, without help

In this sentence it adds an important contrast:

• first, the guide helped
• then, the daughter tried without help

So ella sola strongly highlights her independence.

Because sola is an adjective agreeing with a feminine singular person:

• mi hija is feminine singular
• ella is feminine singular
• so the adjective is sola

Compare:

• él solo = by himself
• ella sola = by herself
• ellos solos = by themselves
• ellas solas = by themselves

Here sola is not the adverb solo meaning only. It is the adjective meaning alone.

Yes, that would also be possible.

• mi hija intentó situar el río sola

This also means my daughter tried to locate the river by herself.

Adding ella gives extra emphasis:

• ella sola sounds a bit more emphatic, almost like she herself, all alone

So both are possible, but ella sola is slightly stronger.

Putting ella sola at the end makes it stand out. It gives the sentence a natural final emphasis:

• ...intentó situar el río ella sola

It is almost like the key final detail is: and she did it on her own.

Spanish word order is flexible, so other positions are possible, but the ending position works very well for emphasis.

Luego means then or afterwards.

It links the two actions in sequence:

1. La guía nos ayudó...
2. y luego mi hija intentó...

So it helps organize the narrative and show that the daughter’s attempt happened after the guide’s help.

In Spain, luego is very common in this meaning.

The accents help show the correct pronunciation.

• guía
• río

In both words, the accent marks that the vowels are pronounced in separate syllables rather than merging into a diphthong.

So:

• guía is pronounced roughly as two syllables
• río is also pronounced as two syllables

Without the accent, the pronunciation pattern would be different.

These are third-person singular preterite forms:

• ayudó
• intentó

The written accent shows that the stress falls on the final syllable.

This is also useful because it distinguishes them from present-tense forms:

• ayudo = I help
• intento = I try

But:

• ayudó = he/she helped
• intentó = he/she tried

So the accent is grammatically important.

Yes. You can also have situarse.

But the meaning changes:

• situar algo = to locate/place something
• situarse = to locate oneself, get positioned, place oneself

Examples:

• Situó el castillo en el mapa = He/She located the castle on the map
• Nos situamos cerca de la puerta = We positioned ourselves near the door

In your sentence, it is not reflexive because the object is el castillo and el río, not the people themselves.