Questions & Answers about Мальчик рисует синими и красными фломастерами.
Why don't we use the preposition с (with) before the markers?
In Russian, when you do something using an object as a tool or instrument, you put that object directly into the Instrumental case without a preposition. Using с here would mean the boy is drawing accompanied by the markers (as if the markers were his friends drawing alongside him!).
Why do the adjectives синими and красными have different endings if they both describe фломастерами?
Both are in the Instrumental plural, but they have different stem types. Красный is a hard adjective, so it takes the hard Instrumental plural ending -ыми. Синий is a soft adjective, so it takes the soft Instrumental plural ending -ими.
What is the dictionary form of фломастерами and how did it get this ending?
The dictionary form is фломастер (marker), which is a regular masculine noun ending in a hard consonant. To put it into the Instrumental plural, you simply add the standard hard ending -ами directly to the noun.
I remember the verb for 'to draw' is рисовать. Where did the -у- in рисует come from?