Foreign Masculines in -us and Indeclinable Borrowings | Czech Grammar

Czech has absorbed a wave of Latin and Greek masculines that arrive wearing the foreign ending -us: cyklus (cycle), rytmus (rhythm), organismus (organism), génius (genius), virus (virus), mýtus (myth). Like the neuter -um nouns, most of them treat the -us as a removable lid: it is stripped off before any Czech ending, and what remains declines on a native pattern. "A cycle" is cyklus, but "of a cycle" is cyklu and "in cycles" is v cyklech — the -us has gone. Set against this group are two others: a handful of fully nativized loans that have swallowed the -us into their stem and decline normally (autobus, kaktus), and a class of indeclinable borrowings (taxi, menu, whisky) that refuse to change at all. This page sorts the three apart.

The -us drop: cyklus declines like hrad

The core pattern: drop the -us, then decline the truncated stem like an ordinary noun. For inanimate nouns like cyklus the model is hrad — hard masculine inanimate. The -us survives only in the nominative/accusative singular; everywhere else it is gone.

Case	Singular	Plural
Nominative	cyklus	cykly
Genitive	cyklu	cyklů
Dative	cyklu	cyklům
Accusative	cyklus (= nom.)	cykly
Vocative	cykle	cykly
Locative	(v) cyklu	(v) cyklech
Instrumental	cyklem	cykly

Apart from the nominative/accusative singular keeping -us, this is hrad exactly: genitive -u, instrumental -em, plural -y/-ů/-ům/-ech/-y. The same goes for rytmus → rytmu, rytmem; organismus → organismu; mýtus → mýtu. The defining move is the deletion: do cyklu, never *do cyklusu.

Celý ten film je o jednom dni, takže je to uzavřený v jednom cyklu.

The whole film is about a single day, so it's contained in one cycle. (cyklus → cyklu, locative)

Bubeník skvěle drží rytmus.

The drummer keeps the rhythm brilliantly. (rytmus, accusative = nominative)

Bez správného rytmu se ta písnička rozpadne.

Without the right rhythm the song falls apart. (rytmus → rytmu, genitive)

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The rule of thumb Czech schoolchildren learn: in a word of more than two syllables, the -us is dropped (cyklus → cyklu, organismus → organismu, romantismus → romantismu). The deletion is automatic. What you must resist is the English-speaker reflex to keep the Latin ending and stack a Czech one on top — *cyklusu, *rytmusu are non-words.

Génius: animate, so it drops -us too — but to -a

When the -us noun denotes a person, it is animate, and the truncated stem follows an animate pattern. Génius ("genius, a brilliant person") drops the -us and behaves like pán in the singular: genitive/accusative génia, dative géniovi, instrumental géniem. The animate accusative génia (= genitive) is the giveaway — a thing would keep its nominative as the accusative, but a person does not.

Case	Singular
Nominative	génius
Genitive	génia
Dative	géniovi / géniu
Accusative	génia (= gen.)
Locative	(o) géniovi / géniu
Instrumental	géniem

So the choice of native pattern follows animacy, just as it does for native nouns: an inanimate -us noun goes hrad (cyklus → cyklu, accusative unchanged), an animate one goes pán/muž (génius → génia, accusative = genitive). Other animate -us/-os names work the same way: Marius → Maria, Pythagoras → Pythagora.

Toho kluka považují za malého génia.

They consider that boy a little genius. (génius → génia, accusative = genitive, animate)

Bez génia toho skladatele by ta opera nevznikla.

Without that composer's genius the opera would never have come about. (génius → génia, genitive)

Virus: the in-between case

A few -us nouns sit on the fence, allowing both the dropped and the retained form. Virus ("virus") is the everyday example: the genitive is viru or virusu, both standard. The dropped viru is the careful, traditional form; virusu is increasingly common and fully acceptable — a sign of a loanword in the middle of being nativized.

Vědci objevili nový kmen viru.

Scientists discovered a new strain of the virus. (virus → viru, dropped form)

Šíření toho viru se zatím nedaří zastavit.

The spread of that virus can't be stopped so far. (virus → viru, genitive)

Nativized loans: autobus keeps its -us

At the far end of the spectrum are loans so thoroughly digested that Czech no longer feels the -us as foreign at all. Autobus ("bus") is the prime case: its -us is treated as part of the stem, and the word declines as a plain hrad-type inanimate with endings added after -us, not in place of it: autobusu, autobusem, autobusy. The same holds for kaktus → kaktusu (cactus) and trolejbus → trolejbusu (trolleybus).

Case	Singular	Plural
Nominative	autobus	autobusy
Genitive	autobusu	autobusů
Dative	autobusu	autobusům
Accusative	autobus (= nom.)	autobusy
Locative	(v) autobuse / autobusu	(v) autobusech
Instrumental	autobusem	autobusy

Jeli jsme tam autobusem, vlak nejel.

We went there by bus, there was no train. (autobus → autobusem, instrumental of means — -us kept)

Na zastávce stála fronta na autobus.

There was a queue for the bus at the stop. (autobus, accusative = nominative)

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How do you know whether a word drops -us or keeps it? Frequency and feel. The learned, bookish words (cyklus, rytmus, organismus, génius) drop it; the everyday, fully domesticated ones (autobus, kaktus, globus) keep it. There is no clean rule — but the safe heuristic is that the more ordinary and high-frequency the word, the more likely it has swallowed its -us.

Fully indeclinable borrowings

A separate class of loanwords does not decline at all: the same form serves every case and number. These are typically words ending in a stressed vowel or an un-Czech vowel sequence that no Czech ending will attach to cleanly: taxi, menu, whisky, atašé (attaché), kupé (compartment), finále (final), tabu (taboo), angažmá (engagement, post). They take their case entirely from the words around them — the preposition, the adjective, the verb.

Přijeli jsme taxíkem... ne, vlastně tím taxi.

We arrived by cab... no, actually by that taxi. (taxi unchanged; case shown by the demonstrative tím)

Co máš na to menu?

What do you say to that menu? (menu unchanged; case carried by na + the demonstrative)

Dal si sklenku whisky.

He had a glass of whisky. (whisky unchanged; genitive sense carried by sklenku)

Hráli jsme až do finále.

We played all the way to the final. (finále unchanged; genitive carried by do)

Because the noun gives no signal, the agreement does all the work. To nové menu (this new menu), toho nového menu (of this new menu) — the noun menu is frozen while the demonstrative and adjective shift case around it. This is exactly the skill of reading agreement rather than the noun. The full inventory and behaviour of these words is on the indeclinable nouns page.

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Beware the false friends. Some words that look indeclinable actually decline: kakao ("cocoa") declines partly (kakaa, kakau, kakaem), and kino ("cinema"), despite its foreign look and final -o, is a perfectly regular m ěsto-type neuter (do kina, "to the cinema"; v kině, "at the cinema"). Don't freeze a word just because it sounds borrowed — most loans in -o decline normally.

Common mistakes

❌ Vrátili jsme se na začátek cyklusu.

Incorrect — the -us is dropped before oblique endings; the genitive is cyklu, never *cyklusu.

✅ Vrátili jsme se na začátek cyklu.

We returned to the start of the cycle. (cyklus → cyklu)

❌ Tu skladbu drží pevný rytmus celou dobu, tančilo se v rytmusu.

Incorrect — the locative drops the -us: v rytmu, not *v rytmusu.

✅ Tančilo se v rytmu té skladby.

We danced to the rhythm of the piece. (rytmus → rytmu)

❌ Považuju ho za opravdového génius.

Incorrect — génius is animate, so the accusative drops -us and equals the genitive: génia.

✅ Považuju ho za opravdového génia.

I consider him a true genius. (génius → génia)

❌ Jeli jsme tam autobem.

Incorrect — autobus is nativized and keeps its -us; the instrumental is autobusem, not a truncated *autobem.

✅ Jeli jsme tam autobusem.

We went there by bus. (autobus → autobusem)

❌ Podívali jsme se na menů.

Incorrect — menu is fully indeclinable; it never takes an ending, the case is shown by the surrounding words.

✅ Podívali jsme se na to menu.

We looked at the menu. (menu unchanged)

Key takeaways

Most -us masculines drop the -us before any oblique ending and decline on a native stem: inanimate cyklus → cyklu, cyklem like hrad; animate génius → génia like pán.
Animacy still decides the pattern: an inanimate -us noun keeps its nominative as the accusative (cyklus); an animate one takes the genitive form for the accusative (génia).
A few sit between forms (virus → viru / virusu), and some are fully nativized and keep the -us (autobus → autobusu, autobusem; kaktus → kaktusu).
Indeclinable loans never change — taxi, menu, whisky, atašé, kupé, finále, tabu — and take their case from the agreement around them. See indeclinable nouns.
Don't over-freeze: kino is a regular neuter (do kina, v kině) and kakao declines partly — a foreign look does not make a word indeclinable. This mirrors the neuter -um drop.

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