Kalamu iliyokosekana iko mezani sasa.

Swahili normally does not use articles like the or a/an.

So kalamu can mean:

• a pen
• the pen

The exact meaning comes from context. In this sentence, the context makes it clear that we mean the pen.

Iliyokosekana is a relative verb form meaning that was missing or which was missing.

It can be broken down like this:

• i- = agreement with kalamu
• -li- = past tense
• -yo- = relative marker, giving the sense of that/which
• kosekana = be missing / be unavailable / not be found

So:

• kalamu iliyokosekana = the pen that was missing

Because in Swahili, a relative clause usually comes after the noun it describes.

So:

• kalamu iliyokosekana literally follows the pattern pen that-was-missing

This is actually similar to English:

• the pen that was missing

So the order is very natural in Swahili.

That i- shows noun class agreement with kalamu.

Kalamu belongs to the N class (often called class 9 in the singular), and words connected to it often take matching agreement prefixes.

So:

• i-li-yo-kosekana = that was missing (agreeing with kalamu)
• i-ko = it is located (also agreeing with kalamu)

This agreement is a very important part of Swahili grammar.

Because iko is used for location.

In Swahili:

• ni is used for identification or classification
  
  • Hii ni kalamu = This is a pen
• iko is used for saying where something is
  
  • Kalamu iko mezani = The pen is on the table

So here, since the sentence is about where the pen is, iko is the correct choice.

Mezani comes from:

• meza = table
• -ni = a locative suffix

So mezani means something like:

• on the table
• at the table
• sometimes in the table in other contexts, though not here

The suffix -ni often covers meanings that in English use different prepositions like in, on, or at.
Here, the natural translation is on the table.

No. -ni is more general than any one English preposition.

Depending on context, it can mean:

• in
• on
• at
• sometimes other location-related meanings

So learners should not try to match -ni to only one English word.
In mezani, English happens to use on the table.

Sasa means now, and putting it at the end is very natural in Swahili.

So:

• Kalamu iliyokosekana iko mezani sasa = The pen that was missing is on the table now

You can sometimes move sasa for emphasis, for example:

• Sasa kalamu iliyokosekana iko mezani

But the version with sasa at the end is a very normal, neutral word order.

Here it is singular, because the agreement shows singular:

• i-li-yo-kosekana
• i-ko

Those forms match singular kalamu.

Interestingly, the noun kalamu itself often looks the same in singular and plural. The agreement tells you the number.

So:

• Kalamu iliyokosekana iko mezani sasa = The pen that was missing is on the table now
• Kalamu zilizokosekana ziko mezani sasa = The pens that were missing are on the table now

Notice how the plural agreement changes to:

• ziliyo...? Actually the normal plural form is zilizokosekana
• ziko

Yes, sometimes learners will also see verbs related to being lost or disappearing, such as potea.

But kosekana often gives the idea of:

• being missing
• not being there
• not being found

So iliyokosekana fits well for the one that had been missing.

A verb like potea can be close in meaning, but it may sound more like got lost or disappeared, depending on context. So kosekana is a good choice here.