Mi amiga es muy sensata y decide descansar cuando siente dolor en la rodilla.

In Spanish, sensata means sensible / reasonable / prudent (someone who makes good, rational decisions).

English sensible is a false friend:

• Spanish sensato/sensata → sensible, prudent, reasonable
• Spanish sensible → sensitive (emotionally or physically), not sensible

So mi amiga es muy sensata means my friend is very sensible / reasonable, not sensitive.

Ser (es) is used for more permanent or defining characteristics:

• Mi amiga es muy sensata = being sensible is part of her character.

Estar would be used for temporary states or conditions:

• Mi amiga está cansada = my friend is (feeling) tired (right now).

Here, sensibleness is a stable trait, so es is correct.

Spanish adjectives must agree in gender and number with the noun:

• amiga → female friend
• sensata → feminine form of the adjective sensato

So:

• Female: mi amiga es muy sensata
• Male: mi amigo es muy sensato

Both noun (amigo/amiga) and adjective (sensato/sensata) change to match the person’s gender.

Use muy with adjectives and adverbs:

• muy sensata = very sensible
• muy alto, muy rápido, etc.

Use mucho/mucha with nouns:

• mucho dolor = a lot of pain
• mucha paciencia = a lot of patience

So with the adjective sensata, the correct intensifier is muy, not mucho/mucha.

In Spanish, the simple present is used both for:

• actions happening now, and
• general truths / habits / routines.

So:

• Mi amiga es muy sensata = she is (in general) very sensible
• decide descansar cuando siente dolor = she decides to rest when she feels pain (whenever that happens).

This is like English present simple: She rests when she feels pain.

The verb decidir is followed directly by an infinitive (no preposition) when it has the same subject:

• decidir + infinitive
  
  • Decide descansar. = She decides to rest.
  • Decidimos salir. = We decide to go out.

You only use que if you start a new clause:

• Decide que va a descansar. = She decides that she is going to rest.

But decide de descansar is incorrect.

No. With decidir, the correct pattern is:

• decidir + infinitive (same subject): decide descansar
• decidir que + clause (maybe different subject): decide que descanse (she decides that he/she should rest)

Prepositions like de or en are not used between decide and the infinitive in this structure.

Descansar is usually not reflexive when it simply means to rest:

• Quiero descansar. = I want to rest.

Descansarse exists but is:

• less common, and
• often used in some regions with a slightly different nuance (more like to take a rest / get some rest).

In standard usage, especially in Latin America, descansar (non‑reflexive) is the normal choice here:

• decide descansar = she decides to rest.

Spanish often omits subject pronouns because the verb ending already shows who the subject is:

• decide → he/she decides
• siente → he/she feels

We know it’s she from mi amiga earlier in the sentence.
Adding ella would sound unnecessary here:

• Mi amiga es muy sensata y ella decide descansar… is grammatical, but more emphatic or repetitive. The natural choice is to omit ella.

Both cuando siente and cuando sienta can be correct, but they are used in different situations:

• Cuando + present indicative (cuando siente)
  Used for habitual or general situations:

  • Descansa cuando siente dolor.
    She rests whenever she feels pain (it’s her habit).

• Cuando + subjunctive (cuando sienta)
  Used for future / uncertain / not yet realized situations:

  • Descansará cuando sienta dolor.
    She will rest when she feels pain (in the future, not yet happening).

In your sentence, we’re describing a general habit, so cuando siente (indicative) is the natural choice.

All are possible, but they have different structures:

1. Siente dolor en la rodilla

   • Literally: she feels pain in the knee
   • Uses sentir
     • dolor (direct object).

2. Tiene dolor en la rodilla

   • Literally: she has pain in the knee
   • Uses tener
     • dolor; also correct, slightly more neutral.

3. Le duele la rodilla

   • Literally: the knee hurts her
   • Very natural and common, especially in speaking.

In everyday Latin American Spanish, le duele la rodilla is often the most idiomatic, but siente dolor en la rodilla is perfectly correct and maybe a bit more formal or descriptive.

With body parts, Spanish usually uses the definite article (el/la) rather than a possessive (mi/tu/su) when it’s clear whose body part it is:

• Siente dolor en la rodilla. = She feels pain in her knee.
• Me duele la cabeza. = My head hurts.

Using su rodilla is grammatically correct, but:

• en la rodilla sounds more natural and standard when the owner is obvious from context.

Both can appear, but they are not used in exactly the same way:

• dolor en la rodilla
  Focuses on the location of the pain: pain in the knee.

• dolor de rodilla(s)
  Can sound more like a type/kind of pain (knee pain in general), or be used in broader statements:

  • Tiene dolor de rodilla desde hace años. = She has had knee pain for years.

In this specific sentence, dolor en la rodilla is the most natural way to express pain in her knee at that moment or when it happens.