Using Wish

English speakers use "wish" to show that they want a situation to be different. The verb after "wish" is one tense back, so that if you are wishing for a different present situation, the tense that follows "wish" is past simple or past continuous. If you are wishing that a past situation was different, the tense that comes after "wish" is past perfect. You can also use "wish" with modal verbs in English.

Wish and present situations
Situation: My sister is untidy. (Verb "to be" in the present simple)
Wish:I wish she was tidier. (Verb "to be" in past simple)

Situation: I am going to London next week. (Verb "to go" in present continuous)
Wish: "I wish I wasn't going to London next week." (Verb "to go" in past continuous)

Situation: I haven't studied for the English test. (Verb "to study" in present perfect)
Wish: "I wish I had studied for the English test." (Verb "to study" in past perfect)

Wish and past situations
Situation: I didn't go on holiday this year. (Verb "to go" is in past simple)
Wish: "I wish I had gone on holiday this year. (Verb "to go" is in past perfect.)

Wish with modal verbs
With could to refer to ability

Situation: I can't play a musical instrument.
Wish: "I wish I could play a musical instrument."

With would to refer to habits and free will

Situation: He whistles in the office.
Wish: I wish he wouldn't whistle in the office. (In this sentence you are stressing the fact he wants to whistle and makes a habit of it.)

You could also say: "I wish he didn't whistle in the office." (In this sentence you aren't stressing his desire to whistle, but you are just making a comment about a present situation.)