Se vi lasos la planton sen akvo, ĝi baldaŭ ne plu estos bela.

Because this sentence is a real future condition, not a hypothetical one.

• Se vi lasos... = If you leave...
• lasos is the future tense of lasi.

In Esperanto, -os is used when something is expected to happen in the future.
You would use -us for a more hypothetical or unreal situation:

• Se vi lasus la planton sen akvo, ĝi ne plu estus bela.
  = If you were to leave the plant without water, it would no longer be beautiful.

So lasos fits a normal, likely future situation.

The -n marks the direct object.

Here:

• vi lasos = you will leave
• la planton = the plant (the thing being left)

So la planton is what receives the action, which is why it takes -n.

Compare:

• La planto kreskas. = The plant grows.
  No -n, because it is the subject.
• Mi akvumas la planton. = I water the plant.
  -n, because it is the object.

La is the definite article, meaning the.

So la planton means the plant, referring to a specific plant already known in context.

If you said planton without la, it would mean a plant or just plant in a less specific sense.

Since the English sentence would normally say the plant, Esperanto uses la here too.

Because sen is a preposition, and nouns after prepositions normally do not take -n.

• sen akvo = without water

The -n ending is mainly for direct objects or direction in some cases, not for ordinary objects of prepositions.

Other examples:

• kun amiko = with a friend
• sub la tablo = under the table
• sen mono = without money

So sen akvo is the normal form.

Because Esperanto often leaves out the article when speaking about something in a general substance or uncountable sense.

• sen akvo = without water

This means the plant is lacking water in general, not necessarily without the water.

You could theoretically say sen la akvo if you meant some specific water already mentioned, but that would be unusual here.

So sen akvo is the natural choice.

Ne plu means no longer or not anymore.

It is made of:

• ne = not
• plu = further, any more, anymore

Together:

• ĝi ne plu estos bela = it will no longer be beautiful

A few useful comparisons:

• ankoraŭ = still
• jam = already
• plu often appears with ne: ne plu = no longer

Examples:

• Mi ne plu laboras tie. = I no longer work there.
• Ŝi ne plu dormas. = She is not sleeping anymore.

Baldaŭ means soon.

It tells you when the result will happen:

• ĝi baldaŭ ne plu estos bela = it will soon no longer be beautiful

So the idea is not just that the plant will stop being beautiful, but that this will happen soon if you do not water it.

In Esperanto, ĝi is the normal pronoun for things, animals of unspecified sex, and sometimes babies.

Since planto is a thing, it is referred to as:

• ĝi = it

So:

• la planto ... ĝi ... = the plant ... it ...

This is completely normal Esperanto.

Because bela is an adjective describing ĝi.

After esti (to be), Esperanto uses an adjective, not an adverb, when describing the subject.

• ĝi estos bela = it will be beautiful

Bele is an adverb, meaning beautifully, and would describe how something is done, not what something is.

Compare:

• La planto estas bela. = The plant is beautiful.
• La planto kreskas bele. = The plant grows beautifully.

So after estos, bela is correct.

The given word order is natural, but Esperanto word order is fairly flexible because grammar endings show the roles clearly.

Standard order here:

• Se vi lasos la planton sen akvo, ĝi baldaŭ ne plu estos bela.

You could move some adverbs around without changing the basic meaning, for example:

• Se vi lasos la planton sen akvo, ĝi ne plu baldaŭ estos bela.

But that version sounds less natural. The original is much better.

So while Esperanto allows flexibility, learners should usually prefer the most straightforward order:

• condition first
• then subject
• then time adverb
• then ne plu
• then verb
• then adjective

Normally, se means if, not when.

• se = if
• kiam = when

So this sentence gives a condition: the bad result happens if you leave the plant without water.

If you said Kiam vi lasos la planton sen akvo..., that would mean When you leave the plant without water..., which sounds more like the speaker assumes it will definitely happen.

So se is the right word for a condition.

Here are the main dictionary forms:

• Se — if
• vi — you
• lasos → lasi — to leave
• la — the
• planton → planto — plant
• sen — without
• akvo — water
• ĝi — it
• baldaŭ — soon
• ne — not
• plu — anymore / any longer
• estos → esti — to be
• bela — beautiful

This is a useful way to analyze Esperanto sentences, because most forms are built very regularly from a base word.

A simple English-speaker-friendly guide is:

Se vi LAH-sos la plahn-TOHN sen AHK-vo, ee-jee bahl-DAHW neh ploo ES-tos BEH-lah.

A few pronunciation notes:

• e is like e in bet
• a is like a in father
• o is like o in for, but cleaner and shorter
• u is like oo in food
• ŭ in baldaŭ sounds like a short w-glide, roughly BAL-dow
• Stress is always on the second-to-last syllable:
  • LA-sos
  • plan-TO-n
  • BAL-daŭ
  • ES-tos
  • BE-la

That regular stress pattern is one of the nice features of Esperanto.