Reflexive pronouns
1. 'se' is a short unstressed form of 'sebe' which is a reflexive personal pronoun in accusative
2. 'si' is a short unstressed form of 'sobě' which is a reflexive personal pronoun in dative
Učím se mluvit česky. - I learn/am learning Czech.
Učím češtinu. - I teach/am teaching Czech.
They are used with a verb that reflects on the subject itself.
- myslet si, posadit se, bát se, umínit si...
The first sentence is with a reflexive pronoun because you learn for/to yourself. The second you teach to someone else.
Another examples:
- Píšu si deník. - I keep a diary. (the words I write in the diary are for myself)
- Co si myslíš? - What do you think? (what is your own personal opinion?)