Ketju irtosi eilen kotimatkalla.

Ketju means chain. In this sentence it is in the nominative singular, which is the basic dictionary form.

It is the subject of the sentence, so ketju is the thing that came off / came loose.

In context, Finnish speakers would often understand this as a bicycle chain, even though the word itself just means chain.

Irtosi is the past tense of the verb irrota.

So:

• irrota = to come loose, to come off, to detach
• irtosi = came loose, came off, detached

This is a very common Finnish past tense pattern. The verb stem changes a little:

• irrotaa is not the form
• the correct dictionary form is irrota
• past: irto-si

The ending -i is part of the past tense formation here.

This is a very important distinction.

• irrota / irtosi is usually intransitive: something comes loose by itself
• irrottaa / irrotti is usually transitive: someone loosens or removes something

So:

• Ketju irtosi = The chain came off / came loose
• Hän irrotti ketjun = He/she removed the chain

In your sentence, the chain is the thing that came loose, so irtosi is the correct form.

Eilen means yesterday, and it is an adverb. Time adverbs in Finnish often appear without any extra case ending.

Other common time words work similarly:

• tänään = today
• huomenna = tomorrow
• eilen = yesterday

So eilen simply tells you when it happened.

Kotimatkalla means on the way home.

It is built from:

• koti = home
• matka = trip, journey, way
• -lla = adessive case ending

So literally it is something like on the home-trip or on the journey home.

In natural English, the best translation is usually on the way home.

The ending -lla / -llä is the adessive case. One of its common uses is to express location or circumstances, often something like:

• on
• at
• during
• in the course of

In expressions like kotimatkalla, the adessive gives the meaning while on the way home or during the trip home.

This is a very common Finnish expression pattern:

• matkalla = on a trip / on the way
• koulumatkalla = on the way to school
• kotimatkalla = on the way home
• työmatkalla = on the way to work / on a work trip, depending on context

Finnish often uses compact compound-like expressions where English would use a prepositional phrase.

Both structure and meaning matter here:

• kotimatkalla is the standard natural expression for on the way home
• kotiin means to home / homeward
• matkalla kotiin is also possible and means roughly the same thing

So you could say:

• Ketju irtosi eilen kotimatkalla.
• Ketju irtosi eilen matkalla kotiin.

Both are natural. The first one is just a very neat, common Finnish way to express it.

No, Finnish word order is fairly flexible, though different orders can change the emphasis.

The neutral order here is:

• Ketju irtosi eilen kotimatkalla.

But you could also say:

• Eilen ketju irtosi kotimatkalla.
• Kotimatkalla ketju irtosi eilen.

These are grammatically possible, but they may sound different in focus or style.

Usually, the original version sounds natural and straightforward: The chain came off yesterday on the way home.

You know because of both meaning and verb agreement structure.

In this sentence:

• ketju is in the basic subject form
• irtosi describes what happened to it

So the sentence is structured as:

• ketju = subject
• irtosi = verb
• eilen = time
• kotimatkalla = circumstance/place-like expression

Finnish does not use articles like the or a, so ketju can mean the chain or a chain, depending on context.

Not absolutely by grammar alone, but in real-life context, very often yes.

The sentence literally just says the chain came off yesterday on the way home. In ordinary conversation, many people would assume this refers to a bike chain, because that is something that commonly comes off while traveling.

So the grammar does not force bicycle chain, but context strongly suggests it.

Sometimes in context it can be translated in a few different ways:

• came off
• came loose
• detached
• got loose

It does not necessarily mean that the chain broke. If it had actually snapped, Finnish would often use another verb, such as one meaning broke.

So irtosi mainly focuses on becoming detached, not on being physically broken.

Finnish has no articles, so it does not have words corresponding directly to English a/an and the.

So:

• ketju can mean a chain or the chain
• context tells you which is more natural

In this sentence, English would usually say the chain, because it sounds like a specific chain already known from the situation.

Yes. Finnish often leaves things unstated if they are understood from context.

The phrase kotimatkalla means on the way home, but it does not by itself specify whose way home. Usually the listener understands that it is the speaker’s or the relevant person’s way home from context.

So English may need to make that more explicit:

• The chain came off yesterday on the way home
• The chain came off yesterday while I was on my way home

Finnish does not always need to spell that out.

A simple pronunciation guide:

• Ketju ≈ KET-yu
• irtosi ≈ IR-to-si
• eilen ≈ EI-len
• kotimatkalla ≈ KO-ti-mat-kal-la

A few useful notes:

• j in Finnish is like English y
• ei sounds roughly like ay in day, but shorter and cleaner
• double consonants and double vowels matter in Finnish, but here there are no especially difficult long vowels except the clear ll in -lla

Stress in Finnish usually falls on the first syllable of each word:

• KEtju
• IRtosi
• EIlen
• KOtimatkalla

A few natural translations are:

• The chain came off yesterday on the way home.
• The chain came loose yesterday on the way home.
• The chain fell off yesterday on the way home.

If the context is a bicycle, came off is probably the most natural everyday choice.