Infiltrația apare când presiunea apei este mare.

Because -a at the end is the Romanian definite article, attached to the noun.

• infiltrație = infiltration / an infiltration
• infiltrația = the infiltration

Romanian usually puts the at the end of the noun instead of before it, unlike English.

So:

• Infiltrația apare... = The infiltration appears / occurs...

Apare is the 3rd person singular present tense of the verb a apărea, meaning to appear, to occur, or to show up.

Here the subject is infiltrația, which is singular, so the verb is also singular:

• infiltrația apare = the infiltration occurs

Useful related forms:

• eu apar = I appear
• tu apari = you appear
• el/ea apare = he/she/it appears
• ele apar = they appear

In this sentence, apare is best understood as occurs rather than literally appears.

Când means when.

So:

• Infiltrația apare când presiunea apei este mare.
• Infiltration occurs when the water pressure is high.

Romanian uses când very much like English when to introduce a time or condition-like clause.

You may also see:

• atunci când = when / at the time when

But in this sentence, simple când is the most natural choice.

Because Romanian uses a genitive relationship here: the pressure of the water / the water pressure.

• presiunea = the pressure
• apei = of the water

So presiunea apei literally means the pressure of the water.

This is a very common Romanian structure:

• ușa casei = the door of the house
• culoarea cerului = the color of the sky
• presiunea apei = the pressure of the water

You cannot say presiunea apă here, because the second noun needs the proper genitive form.

Because apei is the genitive/dative singular form of apă.

Basic form:

• apă = water

Genitive/dative form:

• apei = of the water / to the water

So:

• presiunea apei = the pressure of the water

This change is part of Romanian noun inflection. English uses a separate word like of, but Romanian often changes the noun form itself.

Because mare here is an adjective meaning big, great, or in this context high.

In Romanian, many adjectives have forms that change for gender and number, but mare is the same in the singular masculine and feminine:

• presiunea este mare = the pressure is high
• peretele este mare = the wall is big

Plural:

• presiunile sunt mari = the pressures are high

So in this sentence:

• este mare = is high

Even though presiunea is feminine, mare is still the correct singular form.

Romanian often uses mare where English uses high, especially with things like degree, intensity, pressure, or quantity.

So:

• presiune mare = high pressure
• temperatură mare can mean high temperature
• viteză mare = high speed

Romanian can use words corresponding to high in other contexts, but with many measurements or intensities, mare is the normal adjective.

So presiunea apei este mare is the natural Romanian way to say the water pressure is high.

The given word order is normal and natural:

• Infiltrația apare când presiunea apei este mare.

But Romanian word order is somewhat flexible. You could also say:

• Când presiunea apei este mare, infiltrația apare.

This puts more focus on the condition first: When the water pressure is high, infiltration occurs.

Both are correct. The original version is just a straightforward neutral statement.

No, in standard Romanian you normally keep the verb a fi (to be) in sentences like this.

So you say:

• presiunea apei este mare

Not:

• presiunea apei mare

Romanian usually needs the copula este/e in this kind of sentence.

In everyday speech, este is often shortened to e:

• presiunea apei e mare

That is very common and natural in spoken Romanian.

The main special letters here are:

• ț in infiltrația = like ts in cats
• ă in când = a short central vowel, somewhat like the a in about
• â in când does not appear here; note that când has â, which is a close central vowel, a sound English does not really have
• i at the end of apei is pronounced, but lightly

A rough pronunciation guide:

• Infiltrația ≈ in-feel-TRA-tsya
• apare ≈ a-PA-re
• când ≈ kɨnd, with a specifically Romanian vowel
• presiunea ≈ pre-syu-NE-a
• apei ≈ A-pey
• este mare ≈ YES-te MA-re

The stress usually falls like this:

• infiltraȚIa
• aPAre
• preSIUnea
• Apei
• MAre

It can sound like a general fact, especially in technical or scientific context.

Even though Romanian uses the definite form infiltrația, that does not always mean one single specific real-world example. In technical language, Romanian often uses the definite form to talk about a phenomenon in general.

So here it can mean something like:

• Infiltration occurs when water pressure is high.

This is similar to how English sometimes uses the in formal definitions, although English more often uses zero article in these general statements.

Yes, but it changes the meaning slightly.

• Infiltrația apare când presiunea apei este mare. = Infiltration occurs when water pressure is high. This sounds more general, definitional, or technical.

• O infiltrație apare când presiunea apei este mare. = An infiltration occurs when the water pressure is high. This sounds more like one instance or one possible case.

So the sentence with infiltrația is better for a general rule or explanation.

You negate the verb with nu:

• Infiltrația nu apare când presiunea apei este mare.

That means:

• Infiltration does not occur when the water pressure is high.

Romanian usually places nu directly before the verb:

• nu apare = does not occur
• nu este = is not

So another possible negation inside the subordinate clause would be:

• Infiltrația apare când presiunea apei nu este mare.
• Infiltration occurs when the water pressure is not high.

Those two negations do not mean the same thing, so placement matters.