Clitics in second position after an initial constituent

Czech word order is famous for being flexible, but second-position clitics are a major exception. This rule is a core feature of Czech syntax and remains remarkably strict even in highly informal speech.

What is a clitic?
Clitics are prosodically weak words that need to 'lean' leftward onto the first syntactic unit. They include:

• Past and conditional auxiliaries (jsem, jsi, jsme, jste, bych, bys, by, bychom, byste). Note that in the 3rd person past tense (he/she/it/they), there is no auxiliary clitic at all.
• Reflexive pronouns (se, si)
• Constant clitics: Short personal pronouns (mi, ti, ho, mu, tě). These have distinct long forms, are always unstressed, and can never start a sentence. (Note that in modern Czech, mě serves as both the short and long form, so it can carry stress and start a sentence, e.g., Mě to nezajímá.)
• Inconstant clitics: Pronouns like nám, vám, nás, vás, to, ji, je. These usually act as clitics, but they can carry stress and occupy the first position if emphasized (e.g., Nám se to líbí.).

Note that past and conditional auxiliaries cannot be negated (the negation attaches to the main verb, e.g., nedělal jsem). Forms like nejsem are negative copulas or passive auxiliaries, which carry stress and are NOT clitics, meaning they are not restricted to the second position.

Finding the Second Position:
The clitic cluster must come after the first logical constituent (a word, a full phrase, or a subordinating conjunction). Compare:

• Včera jsem mu to dal. - Yesterday I gave it to him. (After an adverb)
• Ten vysoký muž se mi líbí. - I like that tall man. (After a full noun phrase)
• Řekl, že by si to koupil. - He said that he would buy it. (After a subordinating conjunction)

Important Exceptions:

• Coordinating conjunctions like a (and), i (and/even), and ale (but) do not count as the first position. The clitic ignores them and attaches to the first constituent after them.
• Vocative phrases (direct addresses) and interjections set off by commas do not count as the first position (e.g., Petře, líbí se mi to.).
• In modern Czech, short personal pronoun clitics cannot follow a preposition. If a personal pronoun is the object of a preposition, the long or stressed form must be used (e.g., Dívám se na tebe, not na tě). A major exception is the pronoun to, which frequently and naturally follows prepositions (e.g., na to, o to).

Internal Order & Clitic Climbing:
When multiple clitics meet, they form a cluster in a strict order. A helpful mnemonic for the object pronouns is 'Person before Thing' (Dative before Accusative). Furthermore, Czech features clitic climbing: clitics that logically belong to an infinitive often 'climb' up to join the main clause's second-position cluster. For example, in Já jsem se mu to snažil vysvětlit, the breakdown is: Já [Pos 1] + jsem [Aux] + se [Reflexive] + mu [Dative] + to [Accusative]. Here, mu and to are objects of the infinitive vysvětlit, but they climb to the main clause's second position.

Mandatory Fusions:
The second-person auxiliary jsi mandatorily fuses with reflexives (jsi + se = ses, jsi + si = sis; writing smál jsi se is strictly incorrect) and certain conjunctions (aby + jsi = abys, kdyby + jsi = kdybys). Note that while žes (že + jsi) is a very common contraction, it is informal; in standard formal written Czech, že jsi is perfectly correct and expected.