Breakdown of Мой второй сын — врач, а третий — инженер.
а
and (contrastive)
врач
doctor
мой
my (мой, моя, моё) / mine
сын
son
Questions & Answers about Мой второй сын — врач, а третий — инженер.
Why does второй end in -ой but третий ends in -ий?
Most ordinal numbers act like regular adjectives. Because the stress in второй falls on the very last syllable, it takes the stressed masculine ending -ой (just like большой). Третий (third) is a bit special. It is the only ordinal number from 1 to 10 that has a "soft" stem, so it takes the ending -ий. Both are in the Nominative case here to describe the masculine noun сын (son).
Why does the sentence use а for "and" instead of и?
The conjunction а is used to show a contrast or comparison between two things, often translating to "whereas" or a contrasting "and/but". Since you are comparing the different professions of two different sons, а is the most natural choice. Using и here would sound like you are just piling on unrelated facts.
Why are there long dashes before врач and инженер?
In Russian, the present tense of the verb "to be" (am/is/are) is almost always dropped. When a sentence states that one noun IS another noun (like "son [is a] doctor"), Russian writing replaces that missing "is" with a long dash (—) to separate the two words.
Is it okay that the word сын (son) is missing in the second half of the sentence?