Mıknatıs asma kilidin arkasındaydı.

arkasında is built from several parts:

• arka = back, rear
• -sı = its (3rd person singular possessive)
• -nda = in/on/at

So:

• arkası = its back / the back of it
• arkasında = at its back, which naturally means behind it

This is a very common Turkish pattern for location words:

• önünde = in front of it
• yanında = beside it
• içinde = inside it
• üstünde = on it
• arkasında = behind it

So Turkish often expresses behind X as something more literal like at X’s back.

Because the phrase is a possessive construction:

• asma kilidin arkası = the back of the padlock
• asma kilidin arkasında = at the back of the padlock / behind the padlock

Here, asma kilidin is the possessor, and arkası is the possessed noun.

This is a very common Turkish structure:

• evin kapısı = the door of the house
• arabanın önü = the front of the car
• masanın altı = the underside of the table
• asma kilidin arkası = the back of the padlock

So kilidin is in the genitive case because it is the thing whose back we are talking about.

Two things happen:

1. The genitive ending here is -(n)in / -(n)ın / -(n)un / -(n)ün, chosen by vowel harmony.
2. The final t of kilit changes to d before a vowel-initial suffix.

So:

• base word: kilit
• genitive suffix: -in
• expected shape: kilit + in
• actual result: kilidin

This consonant change is common in Turkish with some words ending in p, ç, t, k:

• kitap → kitabı
• ağaç → ağacı
• kanat → kanadı
• kilit → kilidi / kilidin

So kilidin is the normal form.

-dı is the past tense form of the copula, giving the meaning was.

So:

• arkasında = behind it
• arkasındaydı = was behind it

More literally, this comes from:

• arkasında idi = it was behind it

In modern Turkish, idi often gets attached to the previous word:

• hasta idi → hastaydı = he/she was sick
• evde idi → evdeydi = he/she was at home
• arkasında idi → arkasındaydı = it was behind it

So -dı here is the past was part.

The y is a buffer consonant.

The word arkasında ends in a vowel, and the past copula form attaches beginning with a vowel-like element, so Turkish inserts y to make pronunciation smoother:

• arkasında + idi
• becomes arkasındaydı

This happens a lot in Turkish when two vowel sounds would clash.

Compare:

• iyi idi → iyiydi
• orada idi → oradaydı
• arkasında idi → arkasındaydı

So the y is there for smooth connection.

This is the normal Turkish order for a sentence like this:

• Mıknatıs = subject
• asma kilidin arkasındaydı = predicate

Turkish usually puts the main predicate at the end. So the location statement comes last.

A very natural breakdown is:

• Mıknatıs = The magnet
• asma kilidin arkasındaydı = was behind the padlock

That final position is very typical in Turkish.

It behaves like a fixed compound meaning padlock.

• kilit = lock
• asma kilit = padlock

Historically, asma is related to asmak, to hang, so the idea is something like hanging lock, but learners should usually treat asma kilit as a set expression meaning padlock.

So in this sentence:

• asma kilidin = of the padlock

The genitive ending goes on the last word of the phrase, which is normal:

• asma kilit → asma kilidin

Turkish does not have a definite article like English the.

So:

• mıknatıs can mean the magnet or a magnet
• asma kilit can mean the padlock or a padlock

The exact meaning depends on context.

In this sentence, if the meaning shown to the learner is The magnet was behind the padlock, that interpretation comes from context, not from a separate word like the.

Yes, that is a very helpful way to think about it, even though its structure is noun-based.

In English, behind is a simple preposition.
In Turkish, the idea is often expressed with a noun plus possessive and case endings:

• arkasında = at its back = behind it

So functionally, yes, it works a lot like a postposition in sentences such as this.

Other common examples:

• masanın üstünde = on the table
• evin önünde = in front of the house
• kutunun içinde = inside the box
• kapının arkasında = behind the door

So although the grammar is different from English, the practical meaning is very similar to a location word like behind.

This is due to vowel harmony.

For -da / -de:

• the last vowel before the suffix is a in arka
• so the locative uses the back-vowel form -da
• result: arkasında

For the past copula -dı / -di / -du / -dü:

• the nearest relevant vowel is again a back vowel
• so the form is -dı
• result: arkasındaydı

So the vowels are not random; Turkish chooses them according to vowel harmony.

Yes. That is grammatically possible.

• arkasında idi
• arkasındaydı

These mean the same thing. The shorter attached form is more natural in everyday modern Turkish.

So:

• Mıknatıs asma kilidin arkasında idi.
• Mıknatıs asma kilidin arkasındaydı.

Both mean The magnet was behind the padlock, but the second is the more usual form.

A useful breakdown is:

• Mıknatıs = magnet
• asma kilidin = of the padlock
• arkasında = at its back / behind it
• -ydı = was

So the structure is roughly:

• Mıknatıs
• [asma kilidin arkasında]
• ydı

Or more literally:

• The magnet
• at the padlock’s back
• was

That literal structure helps explain why Turkish uses kilidin and arkasında the way it does.