Det vi saknade var bara en ren kniv till melonen.

Here det does not mean a concrete it in the usual sense. In det vi saknade, it helps form the idea what we lacked.

So:

• det vi saknade = what we lacked
• more literally: that which we lacked

This is a very common Swedish pattern. The whole phrase det vi saknade acts as one unit, and that whole unit is the subject of the sentence.

You often can include som here:

• Det som vi saknade var bara en ren kniv till melonen.

That version is also correct.

In everyday Swedish, som is often left out in this kind of structure, especially in phrases like:

• det vi behöver = what we need
• det jag såg = what I saw
• det hon sa = what she said

So det vi saknade is a shorter, very natural way of saying det som vi saknade.

Because vi saknade is inside a subordinate/relative-type clause, not the main clause.

Swedish main clauses usually follow the verb-second pattern, but subordinate clauses do not. So inside det vi saknade the normal order is:

• vi = subject
• saknade = verb

That is why you get:

• det vi saknade

and not:

• det saknade vi in this structure

Note that det saknade vi could exist in a different sentence pattern, but then it would mean something more like that, we lacked with different emphasis.

Saknade is the past tense of sakna.

• sakna = to miss / to lack
• saknade = missed / lacked

In this sentence, lacked is the best fit, because the object is a thing, not a person.

Examples:

• Jag saknar dig. = I miss you.
• Vi saknade en kniv. = We lacked a knife.

Swedish past tense verbs do not change for person, so:

• jag saknade
• du saknade
• vi saknade

all use the same verb form.

Var is the past tense of är / vara.

• är = is / are
• var = was / were

So:

• Det vi saknar är ... = What we lack is ...
• Det vi saknade var ... = What we lacked was ...

Even though English says what we lacked was, Swedish also uses the singular var here because the subject is the whole idea det vi saknade, treated as one thing.

Bara means only or just.

In this sentence, it shows that the missing thing was not something big or complicated:

• var bara en ren kniv = was just / only a clean knife

So the sense is:

• The only thing we were missing was a clean knife.

The placement is natural Swedish: var bara en ren kniv.

Here ren means clean.

So:

• en ren kniv = a clean knife

The adjective form ren is used because kniv is:

• a common-gender noun (en word)
• singular
• indefinite

In Swedish, adjectives change form depending on the noun. Compare:

• en ren kniv = a clean knife
• ett rent bord = a clean table
• rena knivar = clean knives

In other contexts, ren can also mean things like pure or sheer, but here clean is clearly the intended meaning.

Because the sentence refers to a clean knife, not a specific previously identified knife.

• en kniv = a knife
• kniven = the knife

Using en makes it indefinite: any suitable clean knife would do.

If you said den rena kniven or kniven, that would suggest a particular knife already known in the conversation.

Here till is better understood as for or for use with, not literal movement to.

So:

• en ren kniv till melonen = a clean knife for the melon

This is a natural Swedish way to say a knife intended for that food or situation. It suggests something like:

• a knife to cut the melon with
• a knife for serving the melon

Swedish often uses till in this practical sense.

Melonen is the definite form: the melon.

That usually means the melon is already known from the situation or context. Maybe there is a specific melon on the table, so both speaker and listener know which one is meant.

Forms:

• en melon = a melon
• melonen = the melon

So till melonen means for the melon / for that melon we are dealing with.

Yes. A simpler, more direct version could be:

• Vi saknade bara en ren kniv till melonen.

That means essentially the same thing: We were only missing a clean knife for the melon.

The original version:

• Det vi saknade var bara en ren kniv till melonen.

puts extra focus on what it was that was missing. It is a bit more structured and slightly more emphatic, like English What we lacked was just a clean knife.