Questions & Answers about ¡No seas egoísta conmigo!
Why is it seas and not eres or sé?
To give a negative command ("don't be") in Spanish, you must use the present subjunctive form of the verb. For the informal tú, the present subjunctive of ser is seas. The affirmative command ("be") would be sé, but negative commands always require the subjunctive.
If I am saying this to a man, does egoísta change to "egoísto"?
No, it stays exactly the same. Egoísta belongs to a group of adjectives ending in -ista (like optimista or idealista) that don't change for gender. Whether you are talking to a man or a woman, it is always egoísta.
Why is it conmigo instead of "con mí"?
When the preposition con (with) is followed by the pronoun mí (me), they always merge into the special single word conmigo. The same thing happens with ti (you) to form contigo.
How would I use the grammar from this lesson to say "Don't be as selfish as him"?
You would use the structure igual de + adjective + que. To make this sentence a comparative of equality, you would say: ¡No seas igual de egoísta que él!