Learn Turkish: Sen üzgünsün, şu çocuk mutlu. - You are sad, that child is happy.

The suffix -sün acts as the verb 'you are'. In Turkish, 'to be' is attached directly to the end of the word. Because the last vowel in üzgün is ü, the 4-way vowel harmony rules change the 'you are' suffix from -sin to -sün.

Do I have to use the word sen (you) if üzgünsün already means 'you are'?

No, you can drop sen and just say Üzgünsün, şu çocuk mutlu. Turkish often drops subject pronouns because the ending on the word (like -sün) already tells us who the subject is. Using sen here just adds emphasis, contrasting how 'YOU are sad' with how the child feels.

Why doesn't mutlu (happy) have an 'is' suffix attached to it?

For the third person singular (he, she, it, or a specific noun like 'that child'), there is no suffix for 'is' in the present tense. The word mutlu stays exactly as it is to mean 'is happy'.

Why use şu instead of o for 'that'?

Both o and şu can mean 'that'. However, şu is often used when you are actively pointing something out to the listener, or drawing their attention to a specific thing right in front of you.