Epäilin ansaa, mutta nokkela ystäväni hoksasi sen.

Yes:

• Epäilin = I suspected
• ansaa = a trap / the trap in the partitive
• mutta = but
• nokkela = clever
• ystäväni = my friend
• hoksasi = noticed / realized / spotted
• sen = it

A very literal structure would be:

• Epäilin ansaa = I suspected a trap
• mutta nokkela ystäväni hoksasi sen = but my clever friend noticed it

So the whole sentence is: I suspected a trap, but my clever friend noticed it.

Because epäillä normally takes a partitive object.

So:

• epäillä jotakin = to suspect something
• epäillä jotakuta = to suspect someone

That is why you get:

• ansa = base form
• ansaa = partitive singular

This is not mainly about whether the action is complete or incomplete; it is mostly about the verb itself. Some Finnish verbs simply govern the partitive, and epäillä is one of them.

The basic pronoun is se, but here the pronoun is the object of hoksasi, so it changes form.

In this sentence, sen is the natural object form because the friend successfully noticed or spotted it as a whole event. This is often called a total object.

So:

• se = it / that as a basic form
• sen = it as the object here

Why not sitä?

• sitä is the partitive form
• partitive objects often suggest something unbounded, incomplete, ongoing, or are required by certain verbs
• with hoksasi in this sentence, sen fits the meaning better: the friend actually noticed it

An important point: the trap is first mentioned as ansaa, but when referred to again, the pronoun does not keep the same case. It takes the case needed in its new role in the second clause.

Finnish often shows possession with a possessive suffix attached to the noun.

Here:

• ystävä = friend
• -ni = my

So:

• ystäväni = my friend

And:

• nokkela ystäväni = my clever friend

Notice that the suffix goes on the noun, not on the adjective:

• nokkela = clever
• ystäväni = my friend

You can also say minun ystäväni, which is more explicit. But ystäväni by itself already means my friend.

Because the verb ending already tells you the subject.

In Finnish, epäilin contains the 1st person singular ending, so it already means I suspected.

That is why Finnish often leaves out subject pronouns when they are clear from the verb.

So:

• minä epäilin = I suspected
• epäilin = also I suspected

Both are possible, but the pronoun is often unnecessary.

They are both in the past tense: the Finnish imperfecti, which usually corresponds to the English simple past.

• epäilin = I suspected
• hoksasi = he/she noticed or my friend noticed

So the sentence describes two past events:

1. I suspected a trap
2. my clever friend noticed it

Hoksata is an everyday Finnish verb that can mean things like:

• to notice
• to realize
• to spot
• to catch on

In this sentence, because the object is a trap, English translations like these all make sense depending on context:

• noticed it
• spotted it
• realized it was there

So hoksasi is a bit more lively and colloquial than a very neutral verb like huomasi.

Yes. Finnish word order is more flexible than English word order.

This sentence has a very natural, neutral order:

• Epäilin ansaa, mutta nokkela ystäväni hoksasi sen.

But other orders are possible if you want different emphasis.

For example:

• Epäilin ansaa, mutta ystäväni hoksasi sen.
  = simpler, without nokkela

• Sen hoksasi nokkela ystäväni.
  = puts more emphasis on it

Even though Finnish allows flexibility, the version in your sentence is a very normal way to say it.

Finnish has no articles, so it does not directly mark the difference between a and the the way English does.

That means ansaa could be translated as:

• a trap
• the trap

Context decides.

In this sentence, English often uses a trap for the first mention and then it after that:

• I suspected a trap, but my clever friend noticed it.

That sounds natural in English, even though Finnish does not explicitly mark a versus the.