Этот процент выше, чем был в прошлом месяце.

Because процент is a masculine singular noun, and этот has to agree with it in gender, number, and case.

• этот = masculine singular
• эта = feminine singular
• это = neuter singular

So:

• этот процент = this percentage / this rate

Russian adjectives and demonstratives must match the noun they describe.

In Russian, процент can mean:

• percent
• percentage
• sometimes rate or share, depending on context

In this sentence, it most naturally means this percentage / this rate.

A learner may expect something more like English percent, but Russian often uses процент in places where English might say percentage.

Выше is the comparative form of высокий (high), and it means higher.

So:

• высокий = high
• выше = higher

Russian often uses this comparative form directly, without a separate word for more.

So:

• Этот процент выше = This percentage is higher

Because in the present tense, Russian normally does not use the verb to be (быть) the way English does.

English:

• This percentage is higher

Russian:

• Этот процент выше

The idea of is is understood automatically.

But in the past tense, Russian does use forms of быть, which is why you see был later in the sentence.

Чем is the standard word used in Russian to introduce a comparison after a comparative word like выше (higher), меньше (less), лучше (better), and so on.

So:

• выше, чем... = higher than...
• лучше, чем... = better than...

In this sentence:

• выше, чем был... = higher than it was...

This is a very common Russian pattern. The full idea is something like:

• Этот процент выше, чем процент был в прошлом месяце.
• or more naturally: Этот процент выше, чем он был в прошлом месяце.

But Russian often leaves out repeated words when they are obvious from context.

So чем был в прошлом месяце literally means something like:

• than it was last month

The subject of был is understood to be the same процент.

Because it refers back to процент, which is masculine.

Past-tense forms in Russian agree with the subject in gender and number:

• был = masculine
• была = feminine
• было = neuter
• были = plural

Since процент is masculine singular, the correct form is был.

Because в plus the prepositional case is commonly used to mean in a period of time, especially with words like месяц, год, неделя, and so on.

Here:

• прошлый месяц = last month
• в прошлом месяце = in last month / last month

The noun changes case:

• месяц → месяце

And the adjective must agree with it:

• прошлый → прошлом

So:

• в прошлом месяце = last month

Usually not in this meaning.

• в прошлом месяце = last month / in the previous month → correct here
• в прошлый месяц sounds unnatural for this meaning

With time expressions like this, Russian normally uses the prepositional case after в:

• в прошлом году = last year
• в следующем месяце = next month
• в этой неделе would be wrong, but на этой неделе is used for this week

So time expressions are not always fully predictable, but for месяц, в прошлом месяце is the normal choice.

Yes, that is also possible in some contexts, and Russian often drops words when the meaning is clear.

For example:

• Этот процент выше, чем в прошлом месяце.

This can also mean:

• This percentage is higher than last month.

However, чем был в прошлом месяце is a bit more explicit because it clearly means:

• higher than it was last month

Including был helps show that the comparison is with the earlier value of the same percentage.

Not completely. Russian word order is more flexible than English word order, although some versions sound more natural than others depending on emphasis.

Neutral word order:

• Этот процент выше, чем был в прошлом месяце.

You might also see:

• В прошлом месяце этот процент был ниже.
• Этот процент в прошлом месяце был ниже.

These shift the focus slightly.

In your original sentence, the word order is natural and neutral for stating a comparison.

Because чем introduces a comparative clause here:

• Этот процент выше, чем был в прошлом месяце.

Russian punctuation usually requires a comma before чем when it introduces this kind of clause.

So the comma is standard and important.

Sometimes learners try that because English often uses bigger or more, but выше is the better choice here.

For numerical levels, rates, percentages, temperatures, prices, and similar values, Russian often uses:

• выше = higher
• ниже = lower

So:

• процент выше = the percentage is higher
• цена выше = the price is higher
• температура выше = the temperature is higher

Больше usually means more / greater in amount or quantity, and it would not be the most natural word in this sentence.

A very literal breakdown would be:

• Этот = this
• процент = percentage / percent
• выше = higher
• чем = than
• был = was
• в прошлом месяце = in last month / last month

So literally:

• This percentage is higher than was in the previous month

But natural English would be:

• This percentage is higher than it was last month.

That extra it is understood in Russian and does not need to be stated.