How to delete everything after every a specified word in vim

so i have this string

fruit banana
fruit apple
fruit grape
planet jupiter
planet mars
planet saturn

And I want to change those string to like this

fruit
fruit
fruit
planet jupiter
planet mars
planet saturn

The question is, how can I do this with a vim?

At least 5 ways to do it:

With a substitution

With substitutions you can do many things (see :help substitute). One is that you can remember the first word with a capture group by enclosing that part with \( and \), matching the rest of the line (but not remembering it) and then replacing the line with the remembered group using \1:

:%s/\(fruit\).*$/\1/

With normal commands applied to range of lines

You could specify the lines you want to affect and give a normal mode command to be applied to each line (see :help :normal):

:1,3norm Sfruit

This means, for lines 1 to 3, apply the normal mode command Sfruit, where S means 'delete lines and start insert' and then you just type what you want on the line (see :help S).

With the global command

You can also apply normal commands to lines matched by some pattern (see :help global):

:g/^fruit/norm wd$

This will apply only to lines with the word 'fruit' appearing at the start (you can imagine more elaborate patterns in other scenarios). The normal mode commands wd$ mean:

w   " move cursor forward one word
d$  " delete until end of line

(although we could have used Sfruit again)

With a macro

Another way would be to record a macro (see :help macro) such as qa0wd$jq (notice it uses the same method as above). You can then replay the macro with @a and repeat it many times with @@, or apply it to all lines with :%norm @a(or you could use a range/pattern to specify the lines like above). This breaks downs as follows:

qa  " start recording macro into the 'a' register
0   " move cursor to the beginning of the line      
w   " move cursor forward one word
d$  " delete until end of line
j   " move cursor down to the next line
q   " finish recording the macro

With block selection

You can also select the words you want by switching to visual block mode with Ctrl+v (see :help visual-block), highlighting the part of the file you don't want with regular vim motions (you can press o to put the cursor on the other/opposite corner of the selection block) and finally pressing d to delete the selected content