Starám se o psa, aby byl spokojený.

Here se is a reflexive particle that belongs to the verb starat se. It usually should not be translated separately as myself.

It is best to learn this as one unit:

starat se o + accusative = to take care of / look after

So in this sentence, se is part of the normal verb pattern.

Because starat se o requires the accusative case.

• pes = nominative singular
• psa = accusative singular

Since pes is a masculine animate noun, its accusative singular changes to psa.

That is a very common question. Czech prepositions often behave differently depending on the expression.

For example:

• mluvit o psovi = to talk about the dog
  Here o takes the locative.
• starat se o psa = to take care of the dog
  Here o takes the accusative.

So the important thing is to learn verb + preposition + case together, not just the preposition by itself.

Aby introduces a purpose clause. In English, that is often so that, in order that, or simply so.

So:

aby byl spokojený = so that he is / may be content

It explains the purpose of taking care of the dog.

After aby, Czech normally uses a special subjunctive-like / purpose construction, not the ordinary present tense.

So Czech says:

• aby byl
  not
• aby je

Even though byl looks like the past tense form was, here it does not express past time. After aby, it helps express something intended, desired, or aimed at: so that he will be / may be.

The subject is understood from the first clause: the dog.

Czech often leaves the subject unspoken when it is clear from context. So byl means he was / he be, where he refers to pes.

Because adjectives in Czech must agree with the noun or pronoun they describe.

Here, spokojený agrees with the implied subject pes, which is:

• masculine
• singular

So the masculine singular form is spokojený.

For comparison:

• pes → aby byl spokojený
• kočka → aby byla spokojená

Yes, that is correct.

But Czech often omits subject pronouns, because the verb ending already shows the person:

• starám se = I take care

So adding já is usually for emphasis or contrast, like I’m the one taking care of the dog.

Because aby byl spokojený is a subordinate clause.

In Czech, subordinate clauses are normally separated by a comma, so the comma here is standard and necessary.