Huomasimme suuren eron energiankulutuksessa.

Huomasimme means we noticed.

It contains both:

• the past tense
• the 1st person plural ending, meaning we

So Finnish does not need a separate word for we here. You could say Me huomasimme, but the pronoun me is usually unnecessary unless you want emphasis.

Because the adjective has to match the noun it describes.

Here, suuren describes eron, and both are in the same form. Finnish adjectives agree with their nouns in:

• case
• number

So:

• suuri ero = a/the big difference
• suuren eron = a/the big difference as an object in this sentence

Also, suuri is one of those words whose stem changes when it is inflected:

• basic form: suuri
• genitive-style stem: suuren

Because ero is the object of the verb huomata.

In this sentence, the object is a complete, countable thing: they noticed one specific large difference. In Finnish, that usually gives you the total object.

For a singular noun, the total object often looks like the genitive:

• ero = difference
• eron = the/a difference as a total object

So:

• Huomasimme eron = We noticed the/a difference

Many learners are told that this is genitive form used as the object, while some grammar explanations call it accusative function with genitive form. For practical learning, the important point is: after a verb like huomata, a singular complete object often ends in -n.

This is about the difference between the total object and the partitive object.

Here:

• suuren eron = a whole, definite, complete difference was noticed

If you used:

• suurta eroa

that would sound more like a partitive object, which is used in situations such as:

• incomplete or ongoing action
• negative sentences
• unbounded amounts

Compare:

• Huomasimme suuren eron. = We noticed a clear/whole difference.
• Emme huomanneet suurta eroa. = We did not notice a big difference.

So in your sentence, suuren eron is the normal choice.

It is built from:

• energia = energy
• kulutus = consumption
• -ssa/-ssä = in

Together:

• energiankulutus = energy consumption
• energiankulutuksessa = in energy consumption

So the last part, -ssa, is the inessive case, often meaning in.

Because Finnish compounds often use a linking form from the first word.

Here the idea is roughly:

• energian kulutus = consumption of energy

When this becomes a single compound word, it is written as:

• energiankulutus

So that n comes from the first part and helps connect the words.

This is very common in Finnish compounds, although not every compound is formed in exactly the same way.

Because in Finnish compounds, usually only the last part gets the case ending.

So:

• energiankulutus = energy consumption
• energiankulutuksessa = in energy consumption

The earlier part energian-/energia- stays inside the compound, and the grammatical ending goes on the final element.

This is a very useful rule for Finnish compounds in general.

Because Finnish has no articles.

English needs words like:

• a
• the

Finnish does not. Whether something is understood as a difference or the difference depends on:

• context
• word order
• what has already been mentioned
• the overall situation

So suuren eron can correspond to either a large difference or the large difference, depending on context.

Because Finnish can leave that to the context.

The phrase:

• ero energiankulutuksessa

means something like:

• a difference in energy consumption

It tells you the area where the difference appeared, but not necessarily the two things being compared. Those may already be known from the surrounding discussion.

If you wanted to make the comparison explicit, Finnish could add more information, for example a phrase meaning between X and Y.

No, Finnish word order is fairly flexible because the case endings show the grammatical roles.

The neutral order here is:

• Huomasimme suuren eron energiankulutuksessa.

But for emphasis, you could move parts around, for example:

• Energiankulutuksessa huomasimme suuren eron.

That puts more focus on in energy consumption.

So word order in Finnish often helps show emphasis or information structure, not just grammar.