The postposition karşı means "against" or "towards". When expressing opposition to an idea, a law, or an action, you must use it with a noun in the dative case (using the -e / -a suffix, and a buffer -y- if the noun ends in a vowel).
Noun + Dative + Karşı
To say "against racism" (ırkçılık), the noun takes the dative suffix (and undergoes consonant mutation: ırkçılığa). To say "against the law" (yasa), it takes the buffer -y- (yasaya).
Saying "I am against..."
You can also use karşı as the main predicate of your sentence by attaching personal "to be" suffixes directly to it. Because karşı ends in a vowel, remember to use the buffer -y- before suffixes like -ım (I am) or -ız (We are).
For example, to say "We are against war", "war" takes the dative (savaşa), and karşı takes the "we are" suffix to become karşıyız.
Advocating for Rights
This lesson also introduces vocabulary for civil rights, equality, and peaceful protests. While you stand against (karşı) injustices, you will often march for (için) positive changes, demand them as direct objects, or join actions to support them.