Complex sentence: In the United States Army, a platoon sergeant is usually a Sergeant First Class and is the senior enlisted member of the platoon, and is the primary assistant and advisor to the platoon leader (and acts as the platoon leader in his or her absence).
Split into simpler sentences: In the United States Army, a platoon sergeant is usually a Sergeant First Class. The platoon sergeant is the senior enlisted member of the platoon, and is the primary assistant and advisor to the platoon leader (and acts as the platoon leader in his or her absence).