Learn Spanish (Latin America): Tú eres el primero y yo soy el décimo.

Primero (first) and tercero (third) only drop their final -o when they come directly before a masculine singular noun, like in el primer día (the first day). Because primero stands alone in this sentence without a noun right after it, it keeps the -o.

How would this sentence change if two women were speaking?

Ordinal numbers act like adjectives, meaning they must match the gender of the person or thing they refer to. If two women were speaking, the sentence would be: Tú eres la primera y yo soy la décima. Notice that the articles (el to la) and the endings (-o to -a) both change.

Why do we use eres and soy instead of estás and estoy?

Both pairs mean 'you are' and 'I am', but we use the verb ser (which gives us eres and soy) to talk about identity, permanent traits, or defining what someone is. Because being the first or tenth is treated as an identifying rank here, we use ser instead of estar.

Why does décimo have an accent mark?