Every sentence and clause must have two components: the subject and the predicate. The subject is the noun, pronoun, or noun phrase that performs the action of the sentence’s main verb. The predicate includes the action (the verb) and all attributes of the action.
In English, subjects almost always come before predicates. But they don’t always come right at the beginnings of sentences. For instance, consider this sentence:
If total national debt across all sectors is calculated, as opposed to only government or household debt, the U.S. is hardly the worst case in the developed world, the McKinsey analysis shows. [New Yorker]
Here the subject is the U.S, which comes 17 words into the sentence. The predicate is is hardly the worst case in the developed world. Total national debt is the subject of its conditional dependent clause, but not of the sentence.
Also, while predicates are usually longer than subjects, this is not always the case—for example:
Now, what appears to be the document on which Faulkner modeled that ledger as well as the source for myriad names, incidents and details that populate his fictionalized Yoknapatawpha County has been discovered. [NY Times]
In this sentence, the subject is everything from Now to has been discovered. Granted, you won’t find many subjects longer than this, but the example helps show that sentences can take many different forms.