Taxa de participare este mică.

Because taxa is the definite form of the noun.

• taxă = a fee / a tax
• taxa = the fee / the tax

Romanian usually puts the definite article at the end of the noun instead of using a separate word like English the.

So:

• taxă de participare = a participation fee
• taxa de participare = the participation fee

de participare means something like for participation or related to participation.

In Romanian, a structure like noun + de + noun is very common for expressing relationships that English often shows with two nouns together:

• taxa de participare = participation fee
• literally, something close to fee of participation

So de participare is describing what kind of fee it is.

Here, participare is a noun.

It comes from the verb a participa = to participate, but in this sentence it functions as a noun meaning participation.

So:

• a participa = to participate
• participare = participation

This is similar to English participate → participation.

este is the verb is, from a fi = to be.

In standard Romanian, you normally do use the verb to be in this kind of sentence:

• Taxa de participare este mică. = The participation fee is low.

You generally do not omit it the way some other languages do.

In everyday speech, people often shorten este to e:

• Taxa de participare e mică.

That is very common and natural.

Because mică agrees with taxa, which is feminine singular.

In Romanian, adjectives must agree with the noun in gender and number.

• masculine singular: mic
• feminine singular: mică
• masculine plural: mici
• feminine plural: mici

Since taxa is feminine singular, the adjective must also be feminine singular:

• taxa ... este mică

Even though mică comes after este, it still agrees with taxa.

Yes, mică literally means small, but Romanian often uses mic / mare where English would prefer low / high.

So with prices, fees, costs, or amounts:

• taxa este mică = the fee is low
• prețul este mic = the price is low
• costurile sunt mici = the costs are low

So this is completely natural Romanian.

A simple approximation is:

TAK-sa de par-ti-ci-PA-re ES-te MEE-kuh

A few useful notes:

• x in taxa sounds like ks
• ce / ci in Romanian sound like che / chi in English spelling terms, so participare has a chi sound in the middle
• ă in mică is the Romanian ă, a short neutral vowel, similar to the a in English sofa

A more careful pronunciation guide would be roughly:

/ˈtak.sa de par.ti.tʃiˈpa.re ˈes.te ˈmi.kə/

Yes, you can, and it would be understood.

• taxa de participare = the usual, standard expression
• taxa pentru participare = more literally the fee for participation

Both are possible, but taxa de participare is the more idiomatic and common fixed phrase for participation fee.

Yes, but Taxa de participare este mică is the most neutral and natural order.

This follows the usual pattern:

subject + verb + adjective

• Taxa de participare = subject
• este = verb
• mică = predicate adjective

You might hear:

• Taxa de participare e mică. — same meaning, more conversational

Other word orders are possible for emphasis, but they are less neutral.

The plural would be:

Taxele de participare sunt mici.

Breakdown:

• taxele = the fees
• sunt = are
• mici = small / low (plural form)

Compare:

• Taxa de participare este mică. = The participation fee is low.
• Taxele de participare sunt mici. = The participation fees are low.

Yes:

O taxă de participare este mică.

Grammatically, that means A participation fee is low, but in real usage it may sound a bit odd unless the context makes sense.

More natural indefinite examples might be:

• Există o taxă de participare. = There is a participation fee.
• Au o taxă de participare mică. = They have a low participation fee.

So the definite form taxa de participare is often more natural when talking about a specific event or situation.