Sanduku hili ndimo tulimoweka barua jana usiku.

ndimo is a fused copular-locative form meaning roughly “it is in (there) that…”. Morphologically:

• ni- = the copula “it is”
• -mo = the locative morpheme “inside”
• The consonant d is inserted for phonological reasons in these fused forms. So: ni + (d) + mo → ndimo. Use it to focus an interior location in a cleft.

They all belong to the same family of focus-copula + locative:

• ndimo = “it is inside (that) …” (interior of something)
• ndipo = “it is at/then (that) …” (a specific place or time point)
• ndiko = “it is there (that) …” (more general or distal place) Here, because we’re talking about the inside of a box, ndimo is the right choice.

In this cleft pattern, the verb in the relative clause carries a matching locative marker to indicate “where.” So you get both:

• ndimo to set up the focused location
• -mo in the verb to mark the verb’s action as happening “in it” This redundancy is normal and expected in standard Swahili clefts.

It can be, but it changes the structure and meaning. For example:

• Sanduku hili ndilo tulilotumia kuweka barua jana usiku. = “This box is the one that we used to put the letter last night.” (Focus on the box as the noun) Your original sentence uses ndimo … tulimoweka … to focus the inside of the box as the place where the action occurred.

Yes. For a neutral sentence without special focus:

• Tuliweka barua katika/kwenye sanduku hili jana usiku. This says the same thing but without the cleft emphasis.

Both are common:

• jana usiku (very common, conversational)
• usiku wa jana (a bit more formal or careful) They’re interchangeable in most contexts.

Yes:

• Jana usiku tuliweka barua katika/kwenye sanduku hili. With the cleft, you can also front time using ndipo:
• Jana usiku ndipo tulimoweka barua katika sanduku hili. (Focusing the time: “It was last night that …”)

Swahili doesn’t use articles (a/the). barua can be interpreted as “a letter” or “the letter” depending on context. If you need to be explicit:

• “a letter”: barua moja
• “the letter” (already known in context): often just barua, or add ile (“that one”) if needed: barua ile

By form, barua (class 9/10) is the same in singular and plural. Context usually disambiguates. If you want to specify:

• one letter: barua moja
• two letters: barua mbili, etc.

Often yes, especially with the sense “put into”:

• … tulimoitia barua … or more commonly keep the structure and say: … tuliitia barua (ndani ya) sanduku hili … However, weka is the neutral, very common “put/place,” and fits perfectly here. tia can sound a bit more like “insert/put into.”

Yes:

• Sanduku hili ambamo tuliweka barua jana usiku. = “This is the box in which we put the letter last night.” Here ambamo means “in which,” matching the interior sense of -mo.