Ni parolas pri scienco.

In Esperanto, the verb itself usually carries the tense, so you do not need an extra helping verb like are for the simple present.
Parolas already means speak / are speaking, depending on context.

So Esperanto says:

• Ni parolas = we speak or we are speaking

The ending -as marks the present tense for verbs in Esperanto.

Some common verb endings are:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command / wish
• -i = infinitive

So:

• paroli = to speak
• parolas = speak / is speaking / are speaking

Esperanto verbs do not change according to person or number. The same verb form is used with I, you, he, she, it, we, they.

For example:

• mi parolas = I speak
• vi parolas = you speak
• li parolas = he speaks
• ni parolas = we speak
• ili parolas = they speak

This is much simpler than English, where you have forms like I speak but he speaks.

Pri is a preposition meaning about, concerning, or regarding.

So paroli pri io means to talk about something.

Examples:

• Ni parolas pri scienco = We are talking about science.
• Ŝi parolas pri la libro = She is talking about the book.

Yes, very often it is learned that way.
A useful pattern is:

• paroli pri io = to talk about something
• paroli al iu = to speak to someone
• paroli kun iu = to speak with someone

So in this sentence, pri is the normal preposition for the topic of the conversation.

Because scienco is not the direct object here. It comes after the preposition pri.

In Esperanto, nouns often take -n when they are direct objects, but after a preposition like pri, you normally do not add -n.

So:

• Ni studas sciencon = We study science.
  Here sciencon is a direct object.
• Ni parolas pri scienco = We talk about science.
  Here scienco is part of a prepositional phrase.

Esperanto has no indefinite article like English a/an. It only has the definite article la.

So:

• scienco can mean science in a general sense
• la scienco means the science, usually a specific or already understood one

In this sentence, scienco is being used generally, so no article is needed.

Yes, but it changes the nuance.

• pri scienco = about science in general
• pri la scienco = about the science, or about science as a specific known thing

You would use la if the listener already knows which science, topic, or concept you mean, or if you are referring to science as a specific institution or field in context.

The ending -o marks a noun in Esperanto.

So:

• scienco = science
• libro = book
• domo = house

This is one of the most important patterns in Esperanto:

• -o = noun
• -a = adjective
• -e = adverb

Because of this, scienco is immediately recognizable as a noun.

A good approximate pronunciation is stsi-EN-tso.

A few helpful points:

• c in Esperanto is always pronounced ts
• s is always s
• stress falls on the second-to-last syllable

So sci-en-co is stressed on EN.

The normal, neutral order is:

• Ni parolas pri scienco

But Esperanto word order is fairly flexible, especially when you want emphasis.

For example, you could also say:

• Pri scienco ni parolas

This would sound more marked, as if you were emphasizing science as the topic. The original sentence is the most neutral and natural version.

It can mean either one. Esperanto present tense often covers both the English simple present and present progressive, depending on context.

So ni parolas may mean:

• we speak
• we are speaking

Usually the situation makes it clear which is meant.