Мы можем выбрать какой угодно фильм, лишь бы он не был слишком скучным.

Какой угодно means any, whatever kind of, or whichever you like.

So какой угодно фильм means any film or whatever film we want.

A close synonym is любой фильм, but какой угодно often sounds a bit more like:

• I don't mind which one
• whichever at all
• any one will do

So the sentence has the feeling of We can choose any film at all...

Because фильм is the direct object of выбрать, so it is in the accusative case.

For a masculine inanimate noun like фильм, the accusative looks the same as the nominative:

• nominative: какой угодно фильм
• accusative: какой угодно фильм

So even though it is an object, the form stays какой угодно фильм.

If the noun were masculine animate, you would see a different form. For example:

• выбрать какого угодно актёра = choose any actor

Лишь бы means as long as, provided that, or the only requirement is that.

In this sentence:

• Мы можем выбрать какой угодно фильм, лишь бы он не был слишком скучным.

the idea is:

• We can choose any film, as long as it isn't too boring.

It introduces a condition that matters more than everything else.

A good way to feel it is:

• Anything is fine — as long as...

Yes, был is formally the past tense of быть, but after expressions like лишь бы, Russian often uses a past-tense form to express a wish, condition, or hypothetical idea.

So лишь бы он не был слишком скучным does not mean as long as he was not too boring in a literal past-time sense.

It means something more like:

• as long as it won't be too boring
• as long as it isn't too boring

This is a very common Russian pattern:

• если бы
• хотел бы
• лишь бы

They often use a past-tense-looking verb form for non-past meanings.

Because the бы is already contained in лишь бы.

So the structure is:

• лишь бы + past-tense form

That is enough.

You do not normally say:

• лишь бы он не был бы слишком скучным ✘

That would sound wrong here.

Because after быть, Russian often uses the instrumental case for the predicate word describing what someone or something is like.

So:

• он был скучным = it was boring
• literally, he/it was boring with скучным in the instrumental

Here скучным agrees with фильм:

• masculine
• singular
• instrumental

This is very common after forms of быть when describing a state or quality.

Он refers to фильм.

Since фильм is:

• masculine
• singular

the pronoun is also:

• он = it/he

In English we say it, but in Russian nouns have grammatical gender, so film is masculine and takes он.

Because выбрать is perfective, and here the speaker means choose/pick one film as a completed action.

• можем выбрать = we can choose / we can pick
• можем выбирать = we can be choosing / we are able to choose with more focus on the process or repeated activity

In this sentence, the idea is a single choice, so выбрать is the natural option.

Слишком means too, not just very.

So:

• слишком скучным = too boring

That is important. The sentence does not mean the film must be exciting or brilliant. It only says it must not be excessively boring.

Compare:

• очень скучный = very boring
• слишком скучный = too boring

Because лишь бы он не был слишком скучным is a subordinate clause.

Russian normally puts a comma before conjunctions and clause-introducing expressions like this.

So the sentence is divided into:

• Мы можем выбрать какой угодно фильм
• лишь бы он не был слишком скучным

The comma marks that boundary.

Yes. Мы можем выбрать любой фильм, лишь бы он не был слишком скучным is also correct.

The difference is mostly one of nuance:

• любой фильм = any film
• какой угодно фильм = any film whatsoever / whichever film you like

Какой угодно can sound a little more emphatic or more conversational in this kind of sentence.

Not completely. Russian word order is fairly flexible, though the original sentence is very natural.

You could also say:

• Мы можем выбрать любой фильм, лишь бы он не был слишком скучным.
• Какой угодно фильм мы можем выбрать, лишь бы он не был слишком скучным.

But the original order is the most neutral and straightforward.

Putting какой угодно фильм earlier can add emphasis, but it may sound less neutral than the original.