Learn Turkish: Sen uzun bir mektup yaz. - You, write a long letter.

Turkish follows a Subject-Object-Verb (SOV) word order. Here, the subject Sen (you) comes first, the entire object uzun bir mektup (a long letter) is in the middle, and the action verb yaz (write) sits at the very end. This is the standard way to build a sentence in Turkish!

Why is the word bir in the middle of uzun bir mektup? In English we say 'a long letter'.

When you have an adjective describing a noun with bir (a/an), Turkish places bir right before the noun, after the adjective. The pattern is always Adjective + bir + Noun. That is why it is uzun bir mektup (literally 'long a letter') instead of bir uzun mektup.

Do I always have to use Sen when giving a command to someone?

No, you don't! Because the verb yaz is already a command directed at 'you', the word Sen is optional. Including it here just adds a little extra emphasis, like saying 'You, write a long letter' instead of just 'Write a long letter'.

Why doesn't the verb yaz have any suffixes attached to it?

In Turkish, the most basic form of a verb (the dictionary root) is also the direct command form for the informal 'you' (sen). To tell one person to write, you just use the bare root yaz with absolutely no endings.