Learn Swedish: Ert barn leker ute. - Your child is playing outside.

Ert is used because the word it describes, barn (child), is an ett-word. When you are talking to multiple people (the plural "you"), the word for "your" changes based on the noun's gender and number: er for en-words, ert for ett-words, and era for plural nouns.

Wait, so does ert mean I am talking to one person or multiple people?

You are talking to multiple people, like saying "you guys' child". The base form for plural "your" is er/ert/era. If you were talking to just one person, you would use the singular "your" for an ett-word: ditt barn.

Barn means both "child" and "children". How do we know it is just one child in this sentence?

We can tell it is singular because of the possessive pronoun ert. If the sentence was about multiple children, the pronoun would have to take the plural form era, making it era barn (your children).

Why doesn't the sentence use a word for "is" before leker?

Swedish does not have a separate "is playing" or "are playing" form (the continuous tense). Instead, the standard present tense covers both. Leker translates to both "plays" and "is playing".