Il semble que la plante manque d’eau.

Here il is an impersonal subject. It does not refer to a specific thing. French often uses il in expressions like:

• il semble que = it seems that
• il faut = it is necessary
• il y a = there is / there are

So Il semble que... works like English It seems that.... The real content comes after que.

Que introduces the second clause: la plante manque d’eau.

So the sentence is built like this:

• Il semble = it seems
• que la plante manque d’eau = that the plant is lacking water

In English, we can sometimes drop that, but in French que is normally required in this structure.

Because it is a conjugated verb, not an infinitive.

The subject of this clause is la plante, which is third person singular, so the verb is:

• la plante manque

The infinitive is manquer. The present-tense form for il / elle / on is manque.

Not in this sentence.

Manquer can mean different things depending on the structure:

• manquer quelqu’un / quelque chose = to miss someone / something
  
  • Je manque ma famille. = I miss my family. (In modern standard French, people more often say *ma famille me manque.)*
• manquer de + noun = to lack, to be short of
  
  • La plante manque d’eau. = The plant lacks water / is short of water

So here manque d’eau means is lacking water, not misses water.

Because after manquer de, French usually uses de to express a lack of something in a general sense:

• manquer d’eau = to lack water
• manquer de temps = to lack time
• manquer de courage = to lack courage

You are talking about an unspecified amount of water, not a particular, clearly identified water.

It becomes d’eau because de shortens before a vowel:

• de + eau → d’eau

That shortening is called elision.

French usually drops the final vowel of certain small words before a word beginning with a vowel or silent h.

So:

• de eau would sound awkward
• French changes it to d’eau

This is very common:

• l’eau from le eau
• j’aime from je aime
• c’est from ce est

This is a good question because manque has the same form in both the present indicative and the present subjunctive.

• Indicative: la plante manque
• Subjunctive: que la plante manque

With il semble que, French can use either mood depending on nuance:

• indicative if the speaker presents it more like a probable observation
• subjunctive if the speaker emphasizes uncertainty or appearance

In this sentence, the form manque does not tell you by itself which mood is intended, because the two forms are identical.

Yes. That is a very natural alternative.

• Il semble que la plante manque d’eau.
• La plante semble manquer d’eau.

Both mean roughly the same thing.

The second version is a little more direct and compact. The first version feels slightly more like It seems that..., with the observation presented as a full clause.

You can, but it changes the meaning slightly.

• La plante manque d’eau. = The plant is lacking water.
  This sounds more definite.
• Il semble que la plante manque d’eau. = It seems that the plant is lacking water.
  This is more cautious. The speaker is giving an impression or observation, not stating it as a certain fact.

So il semble que adds a sense of appearance, uncertainty, or inference.

A simple pronunciation guide is:

Il semble que la plante manque d’eau
eel sambl kuh la plant monk doh

A few notes:

• il semble: the l in il is normally heard
• semble has a nasal vowel, roughly like sahmbl
• plante also has a nasal vowel
• manque sounds like monk
• d’eau sounds like doh

There is usually no strong pause in the middle; the sentence flows naturally as one unit.