Я поливаю саджанець лійкою.

Because саджанець is the direct object of поливаю: it is the thing being watered.

In Ukrainian, a masculine inanimate noun in the accusative singular usually looks the same as it does in the nominative singular.

So:

• саджанець = nominative singular
• саджанець = accusative singular

That is why the form does not change here.

Лійкою is the instrumental case of лійка.

The instrumental case is often used to show the tool, means, or instrument used to do something. In this sentence, the watering can is the instrument used for watering.

So:

• лійка = watering can
• лійкою = with a watering can

This is very common in Ukrainian:

• писати ручкою = to write with a pen
• їсти ложкою = to eat with a spoon
• поливати лійкою = to water with a watering can

Поливаю is present tense, first person singular: I water / I am watering.

It comes from the verb поливати, which is imperfective.

So я поливаю can mean:

• I am watering (right now)
• I water (habitually, depending on context)

Ukrainian present tense often covers both English simple present and present continuous.

Yes, it can often be omitted.

Because the verb form поливаю already tells you the subject is I, Ukrainian does not always need я.

So both are possible:

• Я поливаю саджанець лійкою.
• Поливаю саджанець лійкою.

Including я can add a little emphasis or clarity, especially if you want to contrast yourself with someone else.

This is a question of aspect, which is very important in Ukrainian.

• поливати = imperfective
  Focuses on the process, repetition, or ongoing action.
• полити = perfective
  Focuses on the completed action.

So:

• Я поливаю саджанець. = I am watering / I water the sapling.
• Я поллю саджанець. = I will water the sapling / I’ll give it water once.

A learner should notice that поливаю describes the action as ongoing or regular, not as a single completed event.

Yes. Ukrainian word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Я поливаю саджанець лійкою.

But other orders are possible, for example:

• Лійкою я поливаю саджанець.
  Emphasis on with a watering can
• Саджанець я поливаю лійкою.
  Emphasis on the sapling
• Поливаю саджанець лійкою.
  More natural in context when I is already understood

The meaning stays basically the same, but the emphasis changes.

It usually means a young planted tree, shrub, or cultivated young plant, so sapling is often a good translation, though seedling may fit in some contexts.

The exact English word depends on what kind of plant is meant.

A useful nuance:

• саджанець often suggests a young plant intended for planting or already planted
• it is not just any plant in general

So in many garden contexts, sapling or young plant is the safest understanding.

The noun endings give you strong clues.

• саджанець ends in a consonant-like masculine pattern, so it is masculine
• лійка ends in -а, which is a very common feminine ending

This matters because gender affects declension:

• саджанець keeps its accusative form like a masculine inanimate noun
• лійка changes to лійкою in the instrumental singular, following a feminine pattern

The ending is -ою, which is a common instrumental singular ending for feminine nouns ending in -а.

So the pattern is:

• лійка → лійкою
• ручка → ручкою
• ложка → ложкою

You may also sometimes see -ою and -ею / -єю with other feminine nouns, depending on the stem and spelling.

It is pronounced roughly like po-ly-VA-yu, with the stress on ва:

• полива́ю

And:

• саджа́нець — stress on жа
• лійко́ю — stress on ко

Stress is important in Ukrainian, so it is worth learning the stressed syllable together with each new word.

Yes.

Because поливаю is present tense imperfective, it can match either:

• I water the sapling with a watering can
• I am watering the sapling with a watering can

Which English version sounds best depends on context:

• if you are describing what you do regularly, I water
• if you are describing what is happening now, I am watering

Ukrainian does not force this distinction in the same way English does.