Learn Icelandic: Sumir einstaklingar eru mjög feimnir.

Sumir is the masculine plural nominative form of the word for "some". It takes this form to match the masculine plural noun einstaklingar (individuals). If the sentence continued to contrast them with "others", you would use the matching masculine plural form aðrir.

How do I know einstaklingar is masculine plural?

The singular form is einstaklingur. The -ur ending is a very common marker for masculine nouns. To make it plural, the -ur drops and is replaced by -ar, giving us einstaklingar.

Why does the adjective for shy (feiminn) change to feimnir?

In Icelandic, adjectives change to agree with the noun they describe in gender, number, and case. Because einstaklingar is masculine, plural, and in the nominative case, the base adjective feiminn drops its singular ending and adds the masculine plural ending to become feimnir.

If I wanted to say "Some women are very shy", how would the words change?

You would need to use the feminine plural forms for the pronoun, noun, and adjective. "Women" is konur (feminine plural). "Some" becomes sumar, and "shy" becomes feimnar. The whole sentence would be: Sumar konur eru mjög feimnar.