El ojo se adaptó a la lentilla en dos días.

Because the usual Spanish verb here is adaptarse a algo, meaning to adapt to or to get used to something.

So:

• adaptar algo = to adapt something
  
  • Adaptaron el diseño. = They adapted the design.
• adaptarse a algo = to adapt to something
  
  • El ojo se adaptó a la lentilla. = The eye adapted to the contact lens.

In this sentence, se is part of the verb adaptarse. It does not mean the eye literally adapted itself in the English reflexive sense. It is just how Spanish expresses this idea.

Because adaptarse normally takes the preposition a.

So the pattern is:

• adaptarse a + noun/pronoun

Examples:

• Me adapté al horario. = I adapted to the schedule.
• Se adaptó a la ciudad. = He/She adapted to the city.
• El ojo se adaptó a la lentilla. = The eye adapted to the contact lens.

So a la lentilla is required by the verb.

Se adaptó is preterite because it describes a completed change: the eye finished adapting.

The phrase en dos días also supports that idea, because it tells you how long it took for the process to be completed.

• se adaptó = it adapted / it got used to it
• se adaptaba = it was adapting / used to adapt

Compare:

• El ojo se adaptó a la lentilla en dos días.
  = The eye adapted to the contact lens in two days.
  Completed result.

• El ojo se adaptaba poco a poco.
  = The eye was adapting little by little.
  Ongoing process, no clear endpoint.

Here en dos días means in two days or within two days: that is, the adaptation was completed after a two-day period.

It often expresses the time needed to achieve something.

Examples:

• Lo hice en una hora. = I did it in an hour.
• Aprendió a nadar en un mes. = He/She learned to swim in a month.
• El ojo se adaptó a la lentilla en dos días. = The eye adapted to the contact lens in two days.

This is different from durante dos días, which would focus more on something happening throughout two days, not on reaching a result.

Spanish often uses the definite article with body parts when the owner is clear from context.

So Spanish prefers:

• Me duele la cabeza. = My head hurts.
• Se lavó las manos. = He/She washed his/her hands.

In your sentence, El ojo can refer to the eye in a general or context-based way, or to a specific eye already understood in the conversation.

Depending on context, Spanish could also say:

• Mi ojo se adaptó a la lentilla en dos días.

But El ojo is natural if the speaker is talking about the eye as the relevant body part rather than stressing ownership.

Grammatically, yes: ojo is singular, so it means eye, not eyes.

Whether that means literally one eye or just a general statement depends on context.

Possible interpretations:

• one specific eye adapted
• the eye, as a body part, adapted
• a general statement about the wearer’s eye

If you wanted to clearly mean both eyes, you would normally say:

• Los ojos se adaptaron a las lentillas en dos días.

But in real usage, singular can sometimes sound natural if the speaker is talking about the eye as a system or the relevant organ in a general way.

In Spain, lentilla is a very common word for contact lens.

So in Spain:

• lentilla = contact lens
• lentillas = contact lenses

You may also hear:

• lente de contacto

That is also correct, but lentilla is especially common in Spain.

A learner should know that vocabulary can vary by region. In many places outside Spain, lente de contacto or lentes de contacto may be more common.

Because lentilla is a feminine noun:

• la lentilla
• una lentilla

The preposition a combines only with el to make al:

• a + el = al
• a + la = a la

So:

• al ojo
• a la lentilla

That is why the sentence has a la lentilla.

Yes. In many contexts, adaptarse a can be translated as either:

• to adapt to
• to get used to

So this sentence could be understood as:

• The eye adapted to the contact lens in two days.
• The eye got used to the contact lens in two days.

The exact English choice depends on style and context. Got used to often sounds more natural in everyday English, while adapted to sounds a bit more formal or technical.

Adaptarse a is very natural here, especially in a medical or practical context.

Another common possibility is acostumbrarse a, which means to get used to:

• El ojo se acostumbró a la lentilla en dos días.

That is also correct and natural. The difference is mainly one of nuance:

• adaptarse a = adapt to, adjust to
• acostumbrarse a = get used to

In this sentence, adaptarse sounds slightly more physiological or neutral, which fits well with an eye adjusting to a contact lens.