The variants you tried won't work.
The verb needs to agree in person and number with the subject.
Since 'el cuaderno' is referenced (not addressed, not self), it is a third person. Since there is only one notebook, the subject is singular third person.
The verb ending for singular third person is '
á', for the verb
estar.
Use the possessive adjective '
mi' in front of '
cuaderno' to say 'my notebook'.
¿Dónde está mi cuaderno? = Where is my notebook?
The possessive adjectives, arranged according to person and number, are:
| mi, mis | nuestro/a, nuestros/as |
| tu, tus | vuestro/a, vuestros/as |
| su, sus | su, sus |
¿Dónde está tu cuaderno? = Where is your notebook?
¿Dónde está nuestro cuaderno? = Where is our notebook?
¿Dónde están nuestros cuadernos? = Where are our notebooks?
The last sentence shows how a plural third-person subject changes the verb ending, from '
á' to '
án', and the possessive adjective ending, from '
o' to '
os'.
No entiendo = I don't understand
(No comprendo = I don't comprehend)
The verb ending, '-o', conveys the subject's person and number, so there's no need to use the subject pronoun '
yo' in front of '
no'.
Soy = I am
Soy no entiendo. = I am I don't understand. (This doesn't make sense.)