Draai die kleppie los.

Because this is a command, or imperative. In Afrikaans, direct commands usually leave out the subject, just like English often does:

• Draai die kleppie los. = Loosen/Unscrew the little valve.

The understood subject is you.

die means the.

A useful thing to know is that Afrikaans uses die for:

• singular nouns
• plural nouns
• all genders

So Afrikaans does not have different words like the for masculine/feminine/neuter nouns.

kleppie is the diminutive form of klep.

So:

• klep = valve / flap
• kleppie = little valve / small flap

Depending on context, English might translate it as little valve, small valve, or sometimes just valve if the smallness is not important in natural English.

That -pie ending is a diminutive ending, used to mean small, little, or sometimes just to sound more natural and everyday.

With klep, the spelling becomes kleppie:

• klep → kleppie

The extra p is just part of the normal spelling pattern when this diminutive ending is added.

Diminutives are very common in Afrikaans, much more common than in English.

Because draai ... los works as a separable verb.

The full verb is:

• losdraai = to loosen by turning / to unscrew

But in a main clause or command, the two parts separate:

• Draai die kleppie los.

So los goes to the end.

This is similar to how some English phrasal verbs work, such as take off or turn on, where one part can move away from the main verb.

In this meaning, the dictionary form is losdraai.

So you would look up losdraai if you wanted the full verb meaning loosen / unscrew.

Compare:

• Main clause / command: Draai die kleppie los.
• Subordinate clause: ... dat jy die kleppie losdraai.
  = ... that you loosen/unscrew the little valve.

In subordinate clauses, the parts usually stay together.

More or less, yes, but that sounds unnatural in English.

A more natural English translation would usually be:

• Loosen the little valve
• Unscrew the little valve

So although the parts are literally turn + loose, the actual meaning is the action of loosening something by turning it.

A rough English-friendly guide would be:

• Draai ≈ like dry
• die ≈ dee
• kleppie ≈ KLEP-ee
• los ≈ loss

So roughly:

• dry dee KLEP-ee loss

That is only an approximation, but it is a useful starting point for an English speaker.

Not always.

Even though kleppie is literally a diminutive, English does not always translate diminutives directly. In many contexts, a natural translation would simply be:

• the valve

rather than:

• the little valve

So the Afrikaans form tells you something about size or tone, but the best English translation depends on context.