Reflexive verbs

The reflexive verbs are created by adding reflexive pronouns SE or SI to the infinitive.

Form: infinitive + SI or SE

"Se" and "si" are not usually translated into English.
In some cases, "se" and "si" can be replaced with SEBE (=a longer version of a reflexive pronoun, it means "oneself.")
There is no rule on how to determine which verb has "se" and which has "si." We must memorize it. However, "se" is more frequent.

Zpívat - to sing
Zpívat si - to sing, to sing for oneself

Posadit - to sit
Posadit se - to sit down, to make oneself sit down.

A reflexive verb is a verb whose direct object is the same as its subject. If the subject is expressed, "se" or "si" moves before the verb.
Myju se. = Myju sebe (celou/celého). = I’m washing myself (= the entire self).
Myju si ruce. = Myju ruce sobě. = I'm washing hands for/to myself.

with modal verb "muset" (have to) + infinitive:
Musím si umýt ruce. (I have to wash my hands.)
Já si musím umýt ruce. (I have to wash my hands.)

Position
"Se" and "si" are usually behind the verb, but that doesn't always have to be true when the reflexive verb stands in a sentence - it can move before the main verb.
Myju si ruce. - I'm washing my hands for myself.
Musím si umýt ruce. - I have to wash hands for myself.

Sometimes it even moves before "muset":
My si musíme umýt ruce" (We have to wash hands for ourselves.)

SIS and SES
"Sis" and "ses" are used for the second person singular (ty), in the past tense and the conditional.

Ty ses smála. (You were laughing.)
Prohlédl sis ty obrázky. (You looked at the pictures.)
Bála by ses. (You would be scared.)
Co by sis dal? (What would you like?)