Learn Icelandic: Þetta eru fimm mínútur, ekki tíu tímar. - These are five minutes, not ten hours.

They belong to different noun groups. Mínúta (minute) is a feminine noun that ends in -a in the singular, and these typically take -ur in the nominative plural. Tími (hour) is a masculine noun ending in -i, which usually takes -ar in the plural.

If the sentence means "These are", why does it start with the singular word þetta?

When pointing things out or introducing them with the verb "to be", Icelandic always uses the neutral singular word þetta (this/these) or það (it/they/those). You don't change þetta to a plural form, even though you must change the verb to the plural form eru (are) to match the plural nouns.

Do the numbers fimm (five) and tíu (ten) need to match the gender of the nouns?

No, they don't! In Icelandic, only the numbers 1, 2, 3, and 4 change their endings to match the gender of the noun they describe. Numbers from 5 onwards have only one form, so you just use the standard words fimm and tíu for both masculine and feminine nouns.

Can tímar also mean "times"?