If you mean the number "one" it is "ett", however if you are talking about saying e.g. "one table", then it depends on the word as there are two different options depending on what word it is (as the Swedish language have two different articles, comparing to English that only has one, "the"). The options you have for nouns are "en" or "ett".
We spell calendar as "Kalender" in Swedish.
Swedish (language) - svenska Swedish (adjective, nationality) - svensk
Cute is "söt" in Swedish.
Brick is "Tegelsten" in Swedish.
leave = "lämna" in swedish
Välkommen (to one person). Välkomna (to several persons).
April spells "Aprill" in swedish.
Cup in Swedish spells "kopp".
Twenty five is tjugofem in Swedish.
The number 15 is pronounced as "femton" in Swedish.
The number 33 is pronounced as "trettiotre" in Swedish.
The abbreviation of and, translated into "och" in swedish is "å"