Pronunciation of "часа"

Question

What is the right way to stress the word часа?

I've seen both ча́са and часа́, is there a special case for each?

Answer 1

What is the right way to stress the word часа?

The "right" one is ча́са, however, the most frequently used combinations, such as, two hours, three hours, four hours and a quarter of an hour all require the stress shift: два / три / четыре / четверть часа́. So this word is a bit tricky.

Answer 2

When we talk about one hour, in the genitive case we use ча́са. For example: "Не прошло и ча́са, как ты пришел". If we talk about several hours, in the nominative case we use часа́. For example: "Три часа́ прошли незаметно".

Answer 3

Wiktionary often has very helpful entries on declension and stress patterns.

At the entry for час, you can see this:

Declension of час (inan masc-form hard-stem accent-c irreg)

After numbers ending with 2, 3 or 4 (except for those ending with 12, 13 or 14), the stress of the genitive singular moves to the last syllable: два часа́, два́дцать три часа́, со́рок четы́ре часа́.

If you click on the Declension pane to reveal its contents, you will see:

            singular      plural
...
genitive    ча́са, часа́△   часо́в
...
△ Irregular.

At the page about stress patterns, you can find this pattern for type c:

                        ... c        ...
In nom/gen/dat/pre sg   ... stem     ...
In acc sg               ... stem     ...
In ins sg               ... stem     ...
In nom pl               ... ending   ...
In gen/dat/ins/pre pl   ... ending   ...
————————————————————————————————————————
example                 ... слой     ... 

You can find a long list of nouns that follow the same stress pattern here. You can find a shorter list of nouns that have an irregular genitive singular here. If you cross-reference these lists, you can discover that the following words follow the same irregular pattern as час, and for the same reasons:

• ряд
• шаг
• шар

Answer 4

"Час" is one of several masculine nouns that retained the old "dual number" pronunciation. Stress shifts from "ча́са" to "часа́" when the word is used in combination with 2, 3, 4, and compound numbers that require declension of 2, 3, and 4 (e.g. 23, 3002, and so on).

Note that in situations when stress shifts to the last syllable the unstressed "а" is pronounced as [i], for example "два часа" is pronounced as [dvɑ ʧisɑ], not as [dvɑ ʧɑsɑ].