rantan to matto ha kuuraabokkusu no yoko ni oita kke?

In this sentence, ランタンとマット is the topic of the sentence, so it takes は.

The more neutral, non-topical version would be:

• ランタンとマットを クーラーボックスの横に 置いたっけ？

That would simply ask, Did we put the lantern and the mat next to the cooler box?

When you use は, you are bringing ランタンとマット up as the topic, something like:

• As for the lantern and the mat, did we put them next to the cooler box?

So は here replaces what might otherwise be を.

Yes. と connects the nouns and means and.

So:

• ランタン と マット = the lantern and the mat

Then the whole combined noun phrase gets one particle:

• ランタンとマットは

This is very normal in Japanese. You do not need to mark each noun separately.

This breaks down like this:

• クーラーボックス = cooler box / ice chest
• の = links one noun to another, often like of
• 横 = side
• に = marks the destination/location of placement

So:

• クーラーボックスの横 = the side of the cooler box
• more naturally in English: next to the cooler box / beside the cooler box

The に is used because 置く is a verb of placement, and Japanese marks the place where something is put with に.

With 置く meaning to place / to put, Japanese usually uses に for the place where something is placed.

• 机の上に置く = put it on the desk
• ドアの横に置く = put it beside the door

で usually marks the place where an action happens, but with placement verbs like 置く, に is the normal choice because it marks the target location.

So in this sentence, クーラーボックスの横に means at the side of / next to the cooler box as the destination of putting.

置いた is the past form of 置く.

This sentence is asking about whether the action was done earlier:

• 置いたっけ？ = Did we put it there again? / Was it that we put it there?

In English, when you ask about whether something was placed somewhere earlier, you also usually use the past:

• Did we put the lantern and the mat next to the cooler box?

So the past tense fits naturally.

っけ is a casual sentence-ending expression used when the speaker is trying to remember something.

It adds a nuance like:

• Was it ... again?
• Did we ... ?
• How did it go again?
• I’m trying to remember...

So 置いたっけ？ is not just a plain question. It sounds like the speaker is checking their memory.

For example:

• 明日だっけ？ = Was it tomorrow again?
• 何て言ったっけ？ = What did they say again?
• ここに置いたっけ？ = Did I put it here again?

It is casual and often used when talking to yourself or asking someone informally.

Not exactly, though both can sound uncertain.

• っけ is used when trying to recall something the speaker feels they should know already.
• かな expresses wondering or uncertainty more generally.

Compare:

• ここに置いたっけ？
  = Did I put it here again?
  Nuance: I’m trying to remember.

• ここに置いたかな？
  = I wonder if I put it here.
  Nuance: more general wondering.

They can overlap in some situations, but っけ has a stronger trying to remember feeling.

Yes, it is casual.

Signs of casual style:

• the verb is in plain past form: 置いた
• the ending っけ is informal
• there is no polite ending like です or ます

A more polite version would need to be rephrased, for example:

• ランタンとマットはクーラーボックスの横に置きましたっけ？

That said, っけ itself is mainly casual, so even with ました, the sentence can still sound conversational rather than fully formal.

The dictionary form is 置く（おく）, which means to put, to place, or to set down.

The change is:

• 置く → dictionary form
• 置いた → past plain form

This is a ku-verb pattern:

• 書く → 書いた
• 聞く → 聞いた
• 置く → 置いた

So if you know 置く, then 置いた is just its past tense.

マット is a katakana loanword, and its exact meaning depends on context. It can mean things like:

• a mat
• a sleeping mat
• a floor mat
• a camping mat

Since the sentence also mentions a lantern and a cooler box, the context feels like camping or outdoor gear, so マット probably means something like a camping mat or ground mat.

Katakana words often depend heavily on context.

Here, の links two nouns:

• クーラーボックス = cooler box
• 横 = side

So クーラーボックスの横 literally means:

• the side of the cooler box

In more natural English, that becomes:

• next to the cooler box
• beside the cooler box

This is a very common pattern in Japanese:

• 家の前 = in front of the house
• 机の下 = under the desk
• 学校の近く = near the school

Both can refer to something like next to, but they are not exactly the same.

• 横（よこ） = side / beside
• 隣（となり） = the neighboring one / right next door

横 focuses more on position at the side. 隣 often suggests something directly adjacent, often as a neighboring item of the same kind or in an ordered set.

For this sentence, クーラーボックスの横 is very natural because it simply describes a location beside the cooler box.

Japanese often leaves out the subject when it is understood from context.

This sentence does not explicitly say I, we, or you. The listener figures that out from the situation.

Because of っけ, it often feels like the speaker is recalling a shared situation, so in English it may come out as:

• Did we put the lantern and the mat next to the cooler box?
• Did I put the lantern and the mat next to the cooler box?

Japanese does this all the time: if something is obvious or not important, it may be omitted.

Yes, potentially.

は can simply mark the topic, but it can also carry a slight contrastive nuance depending on context.

So this sentence could mean just:

• As for the lantern and the mat, did we put them next to the cooler box?

But in some contexts it could also suggest:

• What about the lantern and the mat—did we put those next to the cooler box?

For example, maybe the speaker is going through several camping items one by one. Then は can sound a little contrastive.

It is a normal Japanese word in the sense that it is commonly used, but it is a loanword written in katakana.

• ランタン = lantern

Japanese uses many katakana words for modern objects, outdoor gear, tools, and imported concepts. In everyday Japanese, ランタン, マット, and クーラーボックス all sound natural.

The structure is:

• ランタンとマットは = as for the lantern and the mat
• クーラーボックスの横に = next to the cooler box
• 置いたっけ？ = did we put them there again? / was it that we put them there?

So the pattern is basically:

• [Topic] は [place] に [past verb] っけ？

This is a common casual pattern for trying to remember a past action or fact.