Usage/Meaning of Instrumental case

• Primary useage is 'with' (in the sense of by means of ) i.e. I wrote with a pencil.
• After 6 prepositions: за (behind/beyond), между (between), над (above), под (under), перед (in front of), с (with)
• Used with parts of day and seasons, when answering question 'when'? i.e. in the morning, in the night, in the fall
• With certain verbs, notably быть . (Normally it's omitted in present tense, but has normal forms in past and future, and instrumental will follow) i.e. Его отец был шахтером
• Note the idiomatic usage of с + instr to join to human subjects. Борис с иваном, шли Борис с Иваном

Some verbs that take instrumental case

• Быть (to be)
• заниматься (to be engaged with something/to study)
• Казаться (to seem)
• Интересоваться (to be interested in something)
• Гордиться (to be proud of)
• Пользоваться (to use)
• Болеть (to be sick with something)
• Командовать (to command)
• Управлять (to control)
• Руководить (to manage/to lead)

How to Decline

Singular

1. ом : masc nouns ending in a consonant, and neuter ending in  о

2. ем/ём : m nouns ending in  ь  or  й , and neuter ending in  е

3. ой : nouns ending in  a , regardless of gender  

4. ей/ёй : nouns ending in  я , regardless of gender (the 10 neuter nouns ending in  мя  are an exception). Use  ёй  if  я  is stressed.  

5. ю : fem nouns ending in  ь , ADD  ю  (not replace)  

6. менем : The 10 neuter nouns ending in  мя (время, имя, пламя, семя, стрёмя, темя, вымя, знамя, бремя, племя)