Each person(s) has its own conjugation just like in English no body else's uses the form am but the form I.
Main mistakes are tend to happen when using plural forms with singular nouns that imply more than one person. Examples:
familia, ropa, cumpleaños, equipo, etc.
Recall that the endings for present tense are: yo (-o), tú (-as, -es), él,ella, Ud. (-a, -e), nosotros (-amos, -emos, -imos), ellos, ellas, Uds. (-an, -en)
Endings for preterit tense are: yo (-é, -í), tú (-aste, -iste), él/ella/Ud (-ó, -ió), nosotros (-amos, -imos), elllos/ellas/Uds (-aron, -ieron)
Endings for imperfect are: yo (-aba, ía), tú (-abas, -ías), él/ella/Ud (-aba, -ía), nosotros (-ábamos, -íamos), ellos/ellas/Uds (-aban, -ían)