Learn French: Il trouve quatorze œufs et quinze pommes au marché. - He finds fourteen eggs and fifteen apples at the market.

No, cardinal numbers from two onwards never change their spelling to match the gender of the noun. Quatorze (fourteen) and quinze (fifteen) stay exactly the same whether they are counting masculine words like œufs or feminine words like pommes. The only number that changes is un (one), which becomes une for feminine words.

How do you pronounce œufs? Does it sound like the singular word œuf?

They sound completely different! In the singular œuf (egg), you pronounce the "f" at the end. But in the plural œufs (eggs), both the "f" and the "s" are completely silent. The plural word sounds similar to the vowel sound in the English word "sir" or "burn" but with rounded lips.

Why does the sentence use au marché instead of à le marché for "at the market"?

In French, the preposition à (at or to) and the masculine word le (the) are never allowed to sit right next to each other. They always combine to form the new word au. Since marché is a masculine word, saying "at the market" must always be au marché.

Why isn't there a word like des (some) before œufs and pommes?