Able to/Unable to

            ABLE TO/UNABLE TO

I --------------------AM,WAS
WE,THEY,YOU, PLURAL----ARE ,WERE
SHE , HE , IT, ANY NAME--IS , WAS

CAN ---IS ABLE TO /AM ABLE TO /ARE ABLE TO
EXPLANATION :
1. First we have to see ,what is given in the question i.e Can or Could
2. If can is given then we have to replace can on the basis of Subject given.
3. If subject is HE,SHE,IT OR ANY NAME then can will be replaced by IS ABLE TO.
4.If subject is WE , YOU, THEY, OR ANY PLURAL then can will be replaced by ARE ABLE TO.
5. If subject is I then AM ABLE TO will be used.

Examples:

 1. Shalini can solve the sums.
Ans: Shalini is able to solve the sums.
2. Prachi can write a letter.
Ans: Prachi is able to write a letter.
3. I can do this.
Ans: I am able to do this.
4. We can play football.
Ans: we are able to play football.

COULD--WAS ABLE TO / WERE ABLE TO
EXPLANATION :
1. First we have to check what is given as a helping verb i.e can or could.
2. If could is given then we will replace could and will use was able to or were able to.
3. If Subject is I, HE , SHE, IT, NAME then we will use was able to in place of could.
4. If subject is YOU, WE, THEY  then we will use were able to in the place of could.
  -
Examples :
1. Boys could read a story.
-> Boys were able to read a story.
2. I could help them.
-> I was able to help them.
3. He could try to make the party grand.
-> He was able to make the party grand.