We first came across to with reversed word order in Jsou to star zeny (They are old women) in which the point of to was hard to grasp, for me. Putting to first helped me to normalise it to English word order and see to as ony/oni they/those more clearly, and the sentence as They/those are old men/women. Putting those with are stopped me looking for an article or extra qualifier with the/those women.
Because 1) R is a "hard consonant". It means that after R is always Y. The only exceptions are loan or foreign words.
2) If we apply this rule, we get word "STARÝ", but it already belongs to singular. So we change R to Ř (which is a "soft consonant" - that means that after Ř is always I. And because "staři" is hard to pronounce, we change "i" to "í"for a better pronunciation.
See the section titled Consonant Shifts in the Tips and Notes for the Plural skill: https://www.duolingo.com/skill/cs/Plural/tips-and-notes. (Currently the Tips are available only in a browser, including a mobile browser, but not in the apps.)