Is the sentence grammatically correct or incorrect. no encuentro mis guantes rojos.

This sentence is correct

(Yo) No encuentro mis guantes rojos

(I cannot find my red gloves)

Note, this sentence does not have explicitly the subject Yo (I), nevertheless it is grammatically correct.

This is because in Spanish (Castilian) is not always necessary that the subject is explicitly in the sentence, this type of subject is called Tacit Subject, Implicit Subject or Null Subject.  

This means the subject is encoded in the verb conjugation.

Also, this sentence is written in negative and in simple present tense with the Spanish verb encontrar (to find), which is conjugated with the subject Yo (I) as encuentro.

In addition, the structure of the sentence is according to the usual basic sentence in Spanish:

Subject + verb + complement

cuando era un niño durante el verano salía a jugar con mis mejores amigos, ¿verdad?

corras is the only one that is informal but doesn't follow the suffix rule. correr is the verb. corre is the formal verb form. corra would be used as the formal is corre ended with an a.

your best bet would be corras, it's the only one that is informal but is still wrong