Antonyms - words belonging to the same part of speech, which differ in spelling and sound and meaning diametrically opposed concepts.
One part of the speech - it is not the only condition under which the opposite meaning of the word can be called antonyms. Between these words must exist a certain common attribute. That is, the two concepts are to describe the feeling, or the time, or space, or the quality and quantity - and in this case they will be antonyms.
Let us examine this definition by examples.
Antonym to the word "First."
Antonym to the word "first" will be the word "now." Both words are adverbs - "when? before "and" when? now". Both of them share a common feature - time description. But if the word "before" describes a situation or event that occurred sometime in the past, the word "now" refers to the present. Thus, the words are opposite in meaning and are antonyms.
Antonym to the word "Friendly".
Antonym for the word "friendly" to the word "unfriendly." Both concepts refer to the same part of speech - adverb. As required by a rule, they are united by a common feature - that is, describe the emotional nuance. But if the word "friendly" means joy and pleasure (for example, by someone's presence), the "unfriendly" has the opposite meaning - one whose appearance or speech characterized by the word, nothing is clearly pleased.
Antonym to the word "tears".
Antonym for the word "tears" will be the word "laughter." And then, and another concept related to the noun, they both describe the emotional impact. But if in the first case, the emotion is clearly negative character - tears of sorrow, tears of sadness, tears of pain - the word "laughter" means joy, happiness and fun. Words are opposite in meaning - and so are antonyms.