Друг поскользнулся на наледи.

Here друг means friend.

Grammatically, it is:

• nominative singular
• masculine
• the subject of the sentence

By itself, друг usually just means friend. In some contexts it can mean a male romantic partner, but that is not the default reading here.

Russian does not have articles.

So Друг поскользнулся на наледи can mean:

• A friend slipped on the ice
• The friend slipped on the ice

The exact choice depends on context. If Russian wants to be more specific, it often uses other words instead, for example:

• мой друг = my friend
• этот друг = this friend

Поскользнулся is:

• past tense
• masculine singular
• perfective
• from the infinitive поскользнуться

It is masculine singular because the subject друг is masculine singular.

Compare:

• друг поскользнулся = a male friend slipped
• подруга поскользнулась = a female friend slipped
• друзья поскользнулись = friends slipped

The -ся is a reflexive postfix.

In this verb, it does not literally mean himself in the English sense. It is simply part of the normal dictionary form поскользнуться, which means to slip.

So you should learn поскользнуться as a whole verb.
Without -ся, you do not get the same standard meaning.

Russian normally uses a perfective verb here because the sentence describes one completed event: the friend slipped once.

So:

• поскользнулся = slipped, had a slip, one finished incident

If you wanted an ongoing or repeated idea, Russian would usually use a different structure or a different verb.

In simple narration, поскользнулся is the natural choice.

Наледи is prepositional singular of наледь.

The preposition на can take different cases:

• accusative for motion onto something
• prepositional for being or happening on something

Here the idea is location: the slipping happened on an icy surface.
So Russian uses на + prepositional:

• на наледи = on the icy patch / on the ice glaze

Наледь is a layer or patch of ice formed on a surface, usually from frozen water.

Depending on context, English translations could be:

• ice
• an icy patch
• a sheet/crust of ice
• sometimes black ice if that fits the situation

It is a fairly specific word, not just the general word лёд in every situation.

Many native speakers do say подскользнулся in everyday speech.

However, the standard literary norm prefers:

• поскользнуться
• поскользнулся

So for learners, поскользнулся is the safest and most standard form to use.

No, Russian word order is fairly flexible.

Друг поскользнулся на наледи is the most neutral order.

Other orders are possible, for example:

• На наледи друг поскользнулся — puts more emphasis on where
• Поскользнулся друг на наледи — can sound more marked or stylistic

Because Russian uses case endings, word order often changes emphasis more than basic grammar.

A rough pronunciation guide is:

друг поскользну́лся на на́леди

Approximate English-style rendering: drook paskalz-NOOL-sya na NA-lye-di

A couple of useful points:

• друг is written with г, but at the end of the word it sounds more like k
• the stress in поскользнулся falls on -ну́-