Sefira - plural or singular

Question

Why when we count the Omer, according to the ArtScroll Siddur, for days 2 - 10, we use the plural ימים, however, from day 11 and on, we switch to the singular and say יום - for example: היום שלשה עשר יום.... לעומר?

Answer 1

For years, days and many other words Tanakh is using the singular form for one object, there is a separate form for two objects, for 3 to 10 it uses the plural form, and reverts back to the singular form above 10. I have found a verse where you can see all of them at once.

Bemidbar 11:19

לֹ֣א י֥וֹם אֶחָ֛ד תֹּֽאכְל֖וּן וְלֹ֣א יוֹמָ֑יִם וְלֹ֣א ׀ חֲמִשָּׁ֣ה יָמִ֗ים וְלֹא֙ עֲשָׂרָ֣ה יָמִ֔ים וְלֹ֖א עֶשְׂרִ֥ים יֽוֹם׃

Bereishit 14:4-5

(...) שְׁתֵּ֤ים עֶשְׂרֵה֙ שָׁנָ֔ה עָֽבְד֖וּ אֶת־כְּדָרְלָעֹ֑מֶר וּשְׁלֹשׁ־עֶשְׂרֵ֥ה שָׁנָ֖ה מָרָֽדוּ׃ וּבְאַרְבַּע֩ עֶשְׂרֵ֨ה שָׁנָ֜ה בָּ֣א כְדָרְלָעֹ֗מֶר וְהַמְּלָכִים֙ אֲשֶׁ֣ר אִתּ֔וֹ

Bemidbar 1:3

מִבֶּ֨ן עֶשְׂרִ֤ים שָׁנָה֙ וָמַ֔עְלָה כָּל־יֹצֵ֥א צָבָ֖א בְּיִשְׂרָאֵ֑ל תִּפְקְד֥וּ אֹתָ֛ם לְצִבְאֹתָ֖ם אַתָּ֥ה וְאַֽהֲרֹֽן׃

There are many other examples at the genealogies (see one from Bereishit 11:12-26). However, this is not a universal rule for all words (שְׁנֵ֣ים עָשָׂ֣ר אֲנָשִׁ֔ים etc.).