La rana si nasconde vicino allo stagno quando l’anatra si avvicina troppo.

Because both are used here as pronominal/reflexive-style verbs:

• nascondersi = to hide / to hide oneself
• avvicinarsi = to approach / to come closer

In this sentence, si refers to the subject:

• La rana si nasconde = The frog hides
• l’anatra si avvicina = the duck comes closer / approaches

In English, we usually do not say hides itself or approaches itself, so the si often does not get translated literally.

They are related, but used differently:

• nascondere = to hide something
  • La rana nasconde il cibo. = The frog hides the food.
• nascondersi = to hide oneself / to hide
  • La rana si nasconde. = The frog hides.

So in your sentence, the frog is not hiding an object; it is hiding itself, which is why Italian uses nascondersi.

Because vicino normally goes with the preposition a:

• vicino a = near / close to

Then a combines with the article:

• a + lo = allo

So:

• vicino a lo stagno → vicino allo stagno

vicino il stagno is not correct here.

Stagno is a masculine singular noun, and because it starts with s + consonant (st-), it takes the article lo:

• lo stagno = the pond

When that article follows a, they contract:

• a + lo stagno → allo stagno

This same pattern happens with other masculine nouns beginning with s + consonant, z, ps, gn, etc.:

• allo zoo
• allo studente
• allo specchio

Because anatra starts with a vowel, and Italian normally shortens la before a vowel:

• la anatra → l’anatra

This is called elision.

So:

• l’anatra = the duck

The apostrophe shows that a vowel has been dropped.

It is about both gender and sound.

Gender

• stagno is masculine
• anatra is feminine

Article form

Italian definite articles change depending on the noun that follows:

• masculine singular:
  • il
  • lo
  • l’
• feminine singular:
  • la
  • l’

So here:

• stagno begins with s + consonant, so it takes lo → lo stagno
• anatra is feminine and begins with a vowel, so la becomes l’ → l’anatra

Because Italian often uses the present tense for:

• habits
• repeated actions
• general truths
• vivid description

So this sentence can mean something like:

• The frog hides near the pond when the duck gets too close
• Whenever the duck comes too close, the frog hides near the pond

It describes a typical or repeated situation.

If you wanted a specific past event, you would use a past tense instead.

Here quando means when and introduces a time clause.

In this sentence, it is followed by the indicative present because the speaker is describing a real or habitual situation:

• quando l’anatra si avvicina troppo

That is completely normal.

If you were talking about a future event, Italian could also use the future:

• La rana si nasconderà quando l’anatra si avvicinerà troppo.

But in your sentence, the present tense gives a general, habitual meaning.

Here troppo is an adverb, not an adjective.

It modifies si avvicina and means:

• too much
• more naturally here: too close

So:

• l’anatra si avvicina troppo = the duck comes too close

Italian often uses troppo with a verb where English may translate it more idiomatically.

Compare:

• Parla troppo. = He/She talks too much.
• Si avvicina troppo. = He/She comes too close.

Italian often leaves out subject pronouns because the verb ending already shows the person and number.

For example:

• si nasconde already tells you it is he/she/it hides
• si avvicina already tells you it is he/she/it approaches

In this sentence, the nouns la rana and l’anatra are stated explicitly for clarity, so there is no need to add pronouns.

English usually needs a subject pronoun, but Italian often does not.

Because vicino a means near / close to, not inside.

• vicino allo stagno = near the pond
• nello stagno = in the pond

So the sentence says the frog hides close to the pond, not necessarily inside it.

That distinction is important:

• vicino a = near
• in / nel / nello = in