Learn Hardcore Finnish: Käännän nämä lauseet itse, vaikka se on välillä vaikeaa. - I translate these sentences myself, even though it is sometimes difficult.

Finnish often uses the partitive singular for predicative adjectives with olla when the subject is abstract or refers to an activity/situation rather than a concrete, countable thing. Here se refers to the activity of translating, so the idiomatic form is se on vaikeaa (it is difficult [to do]). With a concrete countable subject you’d use the nominative: Tehtävä on vaikea (the task is difficult).

Could I say vaikka se on vaikea?

Only if se clearly refers to a specific, countable noun that has been mentioned (e.g., a text, task, exercise). In this sentence, se points to the activity of translating, so vaikeaa is the natural choice. Vaikka se on vaikea would sound off here.

What does välillä mean here? Is it the same as joskus?

Here välillä means “at times, sometimes.” It’s close to joskus and toisinaan. Nuance:

välillä suggests something that happens at intervals or from time to time.
joskus is a bit broader: sometimes/ever/at some point. Note that välillä also means “between” when used as a postposition (e.g., A:n ja B:n välillä), but that’s a different use.

What does se refer to in vaikka se on välillä vaikeaa? Can I omit it?

Se refers to the whole idea of translating these sentences—the situation/activity just mentioned. You can omit it and say vaikka välillä on vaikeaa, which is also grammatical and slightly more impersonal. Using se ties the difficulty more explicitly to the previously mentioned activity.

Does lause mean “sentence” or “clause”?

In everyday Finnish, lause is commonly used for “sentence.” In linguistics, lause is “clause” and virke is the full orthographic “sentence.” In normal usage, saying nämä lauseet for “these sentences” is perfectly fine.

Why is it itse and not itseni? And how is itse different from yksin?

itse is an emphatic pronoun meaning “myself” (as in “I myself”). It emphasizes who does the action.
itseni is the reflexive form used when “myself” is the object: Pesin itseni (I washed myself).
yksin means “alone.” Käännän nämä lauseet itse = I’m the one doing it; Käännän nämä lauseet yksin = I’m doing it without help. You can combine emphasis: ihan itse = all by myself.

Can I move itse to other positions? Does the meaning change?

Yes; all of the following are correct, with subtle differences in emphasis:

Käännän nämä lauseet itse. Neutral-to-strong emphasis on “myself,” especially because it’s clause-final.
Käännän itse nämä lauseet. Slightly highlights that it’s specifically I who will translate these particular sentences.
Minä itse käännän nämä lauseet. Strong focus on “I myself (not someone else).”
Itse käännän nämä lauseet. “I personally will translate these sentences.”
The core meaning stays the same; word order tweaks focus.

Why is it nämä lauseet and not näitä lauseita as the object?

Object case in Finnish depends on completeness/totality:

nämä lauseet = total/complete object (a specific, delimited set you intend to finish). In the plural, the total object looks like the nominative.
näitä lauseita = partitive object (an indefinite amount, or ongoing/incomplete activity), e.g., Käännän näitä lauseita = I’m (in the process of) translating these sentences / some of these sentences.

What time reference does käännän have? Is it present or future?

Finnish has no separate future tense. Käännän (present) can mean present, near future, or habitual action. Context decides:

Now: “I’m translating…”
Future/intention: “I will translate…”
Habitual: “I (regularly) translate…”
Here it’s most naturally read as intention/commitment.

What does vaikka contribute here? How would “even if” be expressed?

Vaikka introduces a concessive idea. With the indicative (vaikka se on) it means “although/even though it is.”
To say “even if,” Finnish uses the conditional: vaikka se olisi välillä vaikeaa (even if it were sometimes difficult).