keeping values of a variable based on the values of another variable

[Maahdi Gholamii]

I have panel data. Each ID in different years(7 waves) has different English, math or science grades; in some waves, they haven't reported it and it's missing. Now I want to keep variable English_grade for each ID only in waves 6 or 7 and get rid of English grades in other waves(They can be reported as missing).
I know the command is not correct but maybe I need sth like that( keep English_grade if Wave==6|7 else drop English_grade). Would you please recommend me a code to solve it?

ID	Wave	English_grade	Math_grade	Sceinec_grade
1	1	75
1	2	.
1	3	.
1	4	84
1	5	.
1	6	.
1	7	88
2	1	.
2	2	.
2	3	55
2	4	70
2	5	.
2	6	69
2	7	.

Thanks,
Mahdi

[Carlo Lazzaro]

Maahdi:
you may want to consider:

. bysort ID: replace English_grade=. if Wave<6
(4 real changes made, 4 to missing)

. list

     +--------------------------------------------+
     | ID   Wave   Englis~e   Math_g~e   Sceine~e |
     |--------------------------------------------|
  1. |  1      1          .          .          . |
  2. |  1      2          .          .          . |
  3. |  1      3          .          .          . |
  4. |  1      4          .          .          . |
  5. |  1      5          .          .          . |
     |--------------------------------------------|
  6. |  1      6          .          .          . |
  7. |  1      7         88          .          . |
  8. |  2      1          .          .          . |
  9. |  2      2          .          .          . |
 10. |  2      3          .          .          . |
     |--------------------------------------------|
 11. |  2      4          .          .          . |
 12. |  2      5          .          .          . |
 13. |  2      6         69          .          . |
 14. |  2      7          .          .          . |
     +--------------------------------------------+

.

[Nick Cox]

keep and drop work on entire observations or variables, so as you say can't be used for your purpose.

What you're asking for is

replace english_grade = . if Wave < 6

Also

if Wave==6|7

is never a way to say

 if Wave==6|Wave == 7

as it means

if (Wave==6)|7

which is not at all what you want.

[Maahdi Gholamii]

Thank you Carlo and Nick for the support!