Laulaja oli energinen koko keikan aikana.

Laulaja means singer. It is gender-neutral.

Finnish usually does not mark gender in words like this, so laulaja can mean a male singer, a female singer, or just a singer when gender is irrelevant. If you really need to specify, Finnish can use words like mieslaulaja for a male singer and naislaulaja for a female singer, but that is often unnecessary.

Oli is the past tense of olla, which means to be.

So:

• on = is
• oli = was

That is why Laulaja oli energinen means The singer was energetic rather than is energetic.

Here energinen is a predicate adjective, describing the subject laulaja after the verb oli.

In this kind of sentence, with a singular subject and olla, the adjective is normally in the basic nominative singular form:

• Laulaja oli energinen. = The singer was energetic.

So energinen is the expected form here.

Keikka usually means a gig, show, or performance. It is a very common word, especially for live music.

It is close to concert, but it often sounds a bit less formal than konsertti. Depending on context, keikka could be:

• a concert
• a gig
• a live set
• a performance

So in this sentence, keikka is probably best understood as gig or show.

Because aikana is a postposition that requires the genitive case.

So:

• basic form: keikka
• genitive form: keikan

That gives:

• keikan aikana = during the gig

There is also consonant gradation here:

• keikka → keikan

The kk becomes k in this form, which is a very common Finnish pattern.

Because aikana is a postposition, not a preposition.

English uses prepositions before the noun:

• during the gig

Finnish often uses postpositions after the noun:

• keikan aikana
  literally something like the gig's during

So the order is normal Finnish word order for this kind of expression.

Koko means whole or entire.

So:

• keikan aikana = during the gig
• koko keikan aikana = during the whole gig / throughout the gig

A useful thing to notice is that koko itself does not change here. The case marking shows up on keikan, not on koko.

Yes. Koko keikan ajan is a very common alternative, and it means almost the same thing.

So both of these work:

• koko keikan aikana
• koko keikan ajan

In many everyday situations, they are nearly interchangeable. Both mean that something lasted throughout the entire gig.

No, Finnish word order is fairly flexible, although this version is a very neutral and natural one.

This sentence is arranged like this:

• Laulaja = subject
• oli = verb
• energinen = description of the subject
• koko keikan aikana = time expression

You could also say:

• Koko keikan aikana laulaja oli energinen.

That puts more focus on the time expression, but the basic meaning stays the same.

Because Finnish does not have articles like English a and the.

So laulaja can mean:

• a singer
• the singer

and the exact meaning depends on context.

The same is true for keikka / keikan. Finnish leaves article-like meanings to context rather than using separate words.