Translation:Don't think about girls all the time, and study languages.
"si" is generally more optional than "se" - "se" forms an integral part of verbs, while "si" often just adds a flavour of "doing something for one own's sake or one own's pleasure".
You can often omit the "si" from "myslet si" and nothing much happens. For example: "Co myslíš ty?" and "Co si myslíš ty?" are equivalent. Similarly, you can begin a sentence with "Myslím, že..." or "Myslím si, že..." (I think that...) and it won't make a difference. However, there are verb phrases with prepositions where we don't have this choice:
- "myslet na +accusative" (to think of/about something) only works without "si" - for example: "Myslíš na Prahu?" - Are you thinking about Prague?
- "myslet si +accusative o +locative" (to think something of/about something) only works with "si" - for example: "Co si myslíš o Praze?" - What do you think about Prague?
So it's good to learn these whole structures - verb + reflexive + preposition + case
You're welcome. About that "flavour" I mentioned, it's best to see in examples:
"Koupím si kabát." - I will buy a coat (for myself). We include "si" to show that the subject will be the beneficiary of the action. Saying "koupím kabát" without "si" raises the question: What will you do with it? Who are you buying it for? Compare with "K narozeninám koupím manželovi kabát" - I will buy my husband a coat for his birthday - we can't use "si" here because there's already another dative object (manželovi). Another comparison: Saying "Koupím dům" (I will buy a house) is more default without the "si" because you're probably buying it for the family, not just for yourself, while "Koupím si dům" specifically states that you will buy a house only for yourself.
Similarly, "Vařím večeři" - I am cooking dinner - means that you're probably cooking more than one portion, not just for yourself, while "Vařím si večeři" means you're only cooking for yourself.
In some situations, it's mandatory. For example: "Myju si ruce a čistím si zuby" - I am washing my hands and brushing my teeth. Leaving these two "si" out begs the question whose hands and teeth you're washing. Here the dative reflexive "si" essentially replaces the possessive "my" that is obligatory in English.