The Sentence – confusing a simple thought

The sentence is the most simple form of expressing a thought. The sentence must have a subject and a predicate – well, not exactly. The subject and predicate can be explicit, or simply understood. We are assuming of course that one wants to express a complete thought. But a sentence may also have an object, may have modifiers, may have connectives, and may have independent elements. So what is so hard about that? Please feel free to help with this understanding.