Instrumental: With and By

The instrumental answers s kým? (with whom?) and čím? (with what? by what?).

• After s/se (with): s mlékem, s kamarádkou, se psem.
• For transport, with no preposition: vlakem, autem, metrem, autobusem, letadlem.
• For professions after stát se, and usually after být (where the nominative is also possible): Stala se herečkou. Chci být lékařem.
• For location after před, za, nad, pod, mezi: před domem, pod postelí.

Endings: masculine and neuter nouns take -em (vlak → vlakem, mléko → mlékem). Feminine nouns in -a take -ou (kamarádka → kamarádkou), and so do masculine nouns in -a (policista → policistou). Feminine nouns ending in a consonant take -í (postel → postelí). In the plural the ending is often -i or -y: s rodiči.