The purpose of a sentence is to express a complete thought.
To be a complete thought, a sentence must contain a subject and a verb.
(A verb is an action word, aka predicate.)
One period at the end of a sentence is quite sufficient, even if the period serves two purposes (or perhaps even more). Two periods would be awkward.
Could you rewrite your question as a full sentence, please? It's not clear what you are asking.
It is 'you are always' and not 'you always' in a sentence for the purposes of correct grammar.
energy and structure
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Control groups are used in experiments for comparison purposes.
Two purposes of a World's Fair are to celebrate great achievements and to address important challenges facing humanity.
Reconnaissance
"While some inhalants are used for medical purposes, others are abused"
Meant to further your (or their) knowledge. For learning. (If you didn't understand my first sentence)
These are two pencils is a correct sentence. it has a subject (pencils) a describer (two) and some sentence support (these are)
The two nouns, 'nouns' and 'sentence' are placed correctly in your sentence.