Wenn die Treppe glatt ist, halte ich mich am Geländer fest.

Because wenn introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• Wenn die Treppe glatt ist
• literally: If the staircase slippery is

This is one of the most important German word-order rules to get used to.

Compare:

• Die Treppe ist glatt. = main clause
• ..., wenn die Treppe glatt ist. = subordinate clause

Because the whole Wenn... clause takes up the first position in the sentence. In German main clauses, the conjugated verb must be in second position.

So the structure is:

1. Wenn die Treppe glatt ist
2. halte
3. ich mich am Geländer fest

That is why you get:

• Wenn die Treppe glatt ist, halte ich mich am Geländer fest.

If you reverse the order, you get the more normal main-clause order:

• Ich halte mich am Geländer fest, wenn die Treppe glatt ist.

Both are correct.

Here, wenn means if.

For a native English speaker, this can be confusing because wenn can also mean when in other contexts.

In this sentence, it expresses a condition:

• Wenn die Treppe glatt ist ... = If the stairs are slippery ...

A useful shortcut:

• wenn = if for conditions
• wenn = when for repeated or general events
• als = when for a single event in the past

Because sich festhalten is commonly used as a reflexive verb here: to hold on.

So German says something like:

• I hold myself fast on/to the railing

That sounds odd in English, but it is natural in German.

The reflexive pronoun changes with the subject:

• ich halte mich fest
• du hältst dich fest
• er hält sich fest
• wir halten uns fest

So mich is there because the subject is ich.

It is a separable verb: festhalten.

That means:

• in a normal main clause, it splits:
  • Ich halte mich am Geländer fest.
• in an infinitive or subordinate structure, it stays together:
  • Ich will mich am Geländer festhalten.
  • ..., weil ich mich am Geländer festhalte.
    (Here only the conjugated form changes, but the idea of separability still matters.)

The base verb is:

• halten = to hold
• festhalten = to hold tight / hold on to

Because festhalten is a separable verb, and in a main clause the prefix goes to the end.

So:

• festhalten → halte ... fest

That is exactly what you see in:

• halte ich mich am Geländer fest

This is very common in German:

• aufstehen → Ich stehe früh auf.
• einkaufen → Wir kaufen heute ein.
• anrufen → Er ruft seine Mutter an.

am is a contraction of an dem.

So:

• am Geländer = an dem Geländer

Geländer is a neuter noun:

• das Geländer

After an in this expression, German uses the dative:

• an dem Geländer
• contracted: am Geländer

So am Geländer means on/at the railing or more naturally in English, onto the railing / to the railing, depending on context.

Because the idiom is sich an etwas festhalten, and here it normally takes the dative.

So you say:

• sich am Geländer festhalten
• sich an der Wand festhalten
• sich an einem Seil festhalten

This is something learners often just need to memorize as part of the verb pattern:

• sich an + dative + festhalten

Even if English might make you expect a different structure, the German pattern is fixed.

Because Treppe is a feminine noun:

• die Treppe = the staircase / the stairs

In this sentence, it is the subject of the clause:

• die Treppe ... ist glatt

So it is in the nominative case, and the feminine nominative singular article is die.

Here glatt means slippery or slick.

Its basic meaning is often smooth, but with floors, stairs, roads, ice, and similar surfaces, it often means slippery.

Examples:

• Die Straße ist glatt. = The road is icy/slippery.
• Der Boden ist glatt. = The floor is slippery.
• Die Treppe ist glatt. = The stairs are slippery.

So the exact English word depends on context.

No. That would be incorrect in standard German.

In a German main clause, the conjugated verb must be in second position. After an opening subordinate clause, the verb comes immediately after that whole clause.

So the correct version is:

• Wenn die Treppe glatt ist, halte ich mich am Geländer fest.

Not:

• Wenn die Treppe glatt ist, ich halte ...

This is a very common mistake for English speakers because English allows If..., I..., but German requires If..., do I... in terms of word order.

Yes. You can put the main clause first:

• Ich halte mich am Geländer fest, wenn die Treppe glatt ist.

This may feel easier to English speakers because the main clause starts in the usual order:

• Ich halte ...

The meaning stays basically the same. The difference is mainly one of focus and style.

• Starting with Wenn die Treppe glatt ist emphasizes the condition first.
• Starting with Ich halte mich am Geländer fest emphasizes the action first.

It is singular here.

• das Geländer = the railing / banister
• die Geländer = the railings

So:

• am Geländer = at/on the railing

In English, railing or banister may sound most natural depending on the staircase.

The dictionary form is:

• sich an etwas festhalten

You can think of it as the full verb pattern, not just festhalten by itself.

Parts of the pattern:

• sich = reflexive pronoun
• an etwas = to/on something
• festhalten = hold on

So when learning it, it is helpful to memorize the whole chunk:

• sich am Geländer festhalten
• to hold on to the railing

That makes it easier to remember both the reflexive pronoun and the preposition.

There are two clauses, and each clause has its own subject:

1. Wenn die Treppe glatt ist

   • subject: die Treppe
   • verb: ist

2. halte ich mich am Geländer fest

   • subject: ich
   • verb: halte

So the sentence has:

• one subject in the subordinate clause: die Treppe
• one subject in the main clause: ich

This is normal in German complex sentences.