A Tagalog sentence may be stated in either the subject-predicate order or the predicate-subject order. The subject-predicate form would normally have the “ay” word in it. This “ay” was considered in the past as the equivalent of the “is/are/was/were” in English, but since it is not needed when a sentence takes the predicate-subject form, it is now considered as simply a linker between the subject and the predicate. In daily conversations, the predicate-subject order seems to be more commonly used than the other.
“Ikaw” is “you” (singular) when it is used as the subject in the subject-predicate sentence order:
IKAW ay matalino. = You are intelligent.
IKAW ba ay kumain na? = Have you eaten already?
This subject, “ikaw” becomes “ka” when it is used in the predicate-subject order. That is, “ikaw” becomes “ka” when it is placed after the predicate.
Matalino KA. = You are intelligent.
Kumain KA na ba? = Have you eaten already?
In interrogative sentences, “ka” usually follows the question word.
Sino KA? = Who are you?
Saan KA pupunta? = Where are you going?
Bakit KA malungkot? = Why are you sad?
Bakit IKAW ang malungkot? = Why is it you who is sad? (Here, “ikaw” is used because the question is about the subject and not the predicate.)