Mi akvumas la planton.

In Esperanto, -as marks the present tense.

So akvumas means water / am watering in the present.

A useful thing for English speakers: Esperanto does not change the verb ending for different subjects.

• mi akvumas = I water / I am watering
• vi akvumas = you water / you are watering
• li akvumas = he waters / he is watering

The verb ending stays -as every time.

It can mean either one.

Esperanto -as does not make the same simple-vs.-continuous distinction that English often does. So:

• Mi akvumas la planton can mean I water the plant
• and it can also mean I am watering the plant

Usually the context tells you which meaning is intended.

The usual Esperanto verb for to water is akvumi.

It comes from:

• akvo = water
• akvumi = to water, to give water to

The -um- part is one of those Esperanto formations that does not have one single fixed meaning. It appears in some words that you mostly learn as vocabulary items.

So for a learner, the important thing here is simply:

• akvo = water
• akvumi = to water

Because la planton is the direct object of the sentence.

In Esperanto, the direct object usually takes the ending -n.

Here:

• Mi = I
• akvumas = am watering / water
• la planton = the plant

The plant is the thing receiving the action, so it gets -n.

Compare:

• La planto kreskas. = The plant grows.
  Here la planto is the subject, so no -n.
• Mi akvumas la planton. = I water the plant.
  Here la planton is the object, so it takes -n.

La is Esperanto’s definite article, meaning the.

So la planton means the plant.

A very important point: Esperanto has no indefinite article. There is no separate word for a or an.

So:

• planto = a plant / plant
• la planto = the plant

In this sentence, la planton suggests a specific plant, probably one that is known from the situation or previous context.

Yes.

Mi akvumas planton is grammatical and would mean something like I am watering a plant or I water a plant.

The difference is:

• la planton = the plant, a specific one
• planton = a plant / plant, not specifically identified

So both are possible, depending on what you want to say.

Yes, Esperanto word order is fairly flexible, partly because the -n ending shows which noun is the object.

The most neutral order is:

• Mi akvumas la planton.

But you could also say:

• La planton mi akvumas.

That puts more emphasis on the plant.

Even so, beginners should usually stick with the normal subject–verb–object order until they feel comfortable.

Mi means I.

It is the subject form, used when the speaker is doing the action:

• Mi akvumas = I am watering

For comparison:

• min = me
• mia = my

So:

• Mi vidas la planton. = I see the plant.
• La hundo vidas min. = The dog sees me.
• Mia planto = my plant

Yes. In Esperanto, plurals take -j, and direct objects still take -n. When both are present, you get -jn.

So:

• la planto = the plant
• la plantoj = the plants
• la planton = the plant, as object
• la plantojn = the plants, as object

So I am watering the plants would be:

• Mi akvumas la plantojn.

No. La never changes.

That is one of the nice simple features of Esperanto.

So you always use la:

• la planto = the plant
• la plantoj = the plants
• la planton = the plant, as object
• la plantojn = the plants, as object

The noun changes, but la stays exactly the same.