Я всегда предупреждаю друга, если на лестнице скользко.

Because друга is the accusative case of друг.

The verb предупреждать / предупредить takes a direct object: you warn someone. In Russian, a masculine animate noun often has the same form in the accusative as in the genitive:

• nominative: друг = friend
• accusative: друга = (I warn) a/the friend

So:

• Я предупреждаю друга = I warn my/the friend

If the noun were inanimate, the accusative would usually look like the nominative instead.

Предупреждаю is the imperfective present form, and it fits the idea of a habitual/repeated action:

• Я всегда предупреждаю друга... = I always warn my friend...

This means whenever this situation happens, I do it.

Предупрежу is perfective future, meaning something like I will warn or I’ll make sure to warn once. That would not match always as naturally here.

So:

• предупреждаю = I warn / I am warning / I usually warn
• предупрежу = I will warn

In Russian, the present tense of an imperfective verb is commonly used for habitual actions, just like in English:

• Я всегда предупреждаю... = I always warn...

It does not mean only right now. It can also mean:

• something you do regularly
• a general habit
• a repeated pattern

So the Russian present tense here works very naturally.

Here если means if:

• если на лестнице скользко = if the stairs are slippery

It introduces a condition.

In real life, this may describe something that happens often, so in English you might sometimes translate it naturally as when depending on context. But grammatically, Russian is using если, which is the normal word for if.

Compare:

• если = if
• когда = when

If the speaker wants to present it as a condition, если is right.

Russian usually says на лестнице for on the stairs / on the staircase.

This is just the normal idiomatic choice with лестница in this meaning. The preposition на is often used for surfaces or areas where something happens:

• на улице = in the street / outside
• на кухне = in the kitchen
• на лестнице = on the stairs / in the stairwell area

After на with location, Russian uses the prepositional case, so:

• лестница → на лестнице

Скользко is a predicative word (often taught as an adverb-like form used in impersonal sentences).

In this sentence, it means:

• it is slippery

Russian often uses this kind of structure without a subject:

• Здесь холодно. = It is cold here.
• Мне грустно. = I am sad.
• На лестнице скользко. = It is slippery on the stairs.

So скользко is not agreeing with any noun here. It is describing the situation/environment, not a specific object.

Because Russian usually does not use the verb to be in the present tense in sentences like this.

English says:

• it is slippery

Russian simply says:

• скользко

So:

• на лестнице скользко = on the stairs, (it is) slippery

This is completely normal Russian grammar.

Yes. Russian word order is more flexible than English word order.

The neutral order here is:

• Я всегда предупреждаю друга, если на лестнице скользко.

But other orders are possible for emphasis, for example:

• Если на лестнице скользко, я всегда предупреждаю друга.
• Друга я всегда предупреждаю, если на лестнице скользко.

These all mean roughly the same thing, but the focus changes slightly:

• starting with если... emphasizes the condition first
• moving друга forward emphasizes the friend

For a learner, the original order is a very natural one to use.

It could be omitted if the subject is clear from the verb:

• Всегда предупреждаю друга, если на лестнице скользко.

Because предупреждаю clearly means I warn, Russian often drops the pronoun я.

However, keeping Я is also perfectly correct. It may sound:

• a little clearer
• slightly more emphatic
• useful if contrasting with someone else

So both are possible.

It can mean different things depending on context.

Russian has no articles, so друга could correspond to:

• a friend
• the friend
• my friend

In many everyday contexts, Я предупреждаю друга will often be understood as I warn my friend, especially if the speaker has a specific friend in mind.

If Russian wants to be more explicit, it can say:

• моего друга = my friend
• одного друга = a certain friend / one friend

But the simple друга is very normal.

Because предупреждать specifically means to warn / to give advance notice, not just to say or to tell.

Compare:

• говорить = to speak / to say
• сказать = to say
• сообщать = to inform
• предупреждать = to warn

So if the idea is I let my friend know because there may be danger, предупреждаю is the best choice.

Here it suggests something like:

• I warn my friend that the stairs are slippery
• I give my friend a heads-up

That nuance is stronger than just I tell my friend.