Beim Abendessen erzähle ich meinem Freund von meinem Tag.

Beim is a contraction of bei dem.

• bei is a preposition that often means at, with, or in some contexts during
• dem is the dative singular form of der/das

So:

• bei dem Abendessen → beim Abendessen

In this sentence, beim Abendessen means something like at dinner or during dinner.

German very often contracts certain preposition + article combinations, and bei dem → beim is one of the most common ones.

In German, all nouns are capitalized, so Abendessen gets a capital A.

Here, Abendessen is being used as a noun meaning dinner / evening meal.

German often turns things that might feel more verb-like in English into nouns. So while English might say at dinner, German says beim Abendessen, with Abendessen treated as a noun.

Because the sentence starts with Beim Abendessen, the verb must still stay in the second position.

German main clauses follow the V2 rule: the finite verb goes in the second position.

So the structure is:

• Beim Abendessen = position 1
• erzähle = position 2
• ich = comes after the verb

That is why you get:

• Beim Abendessen erzähle ich meinem Freund von meinem Tag.

If the sentence started with ich, then it would be:

• Ich erzähle meinem Freund beim Abendessen von meinem Tag.

Both are grammatically possible, but the original sentence puts emphasis on when it happens.

Because erzählen often takes the person receiving the information in the dative case.

So:

• mein Freund = nominative
• meinem Freund = dative

In this sentence, my friend is the person I am telling something to, so German uses the dative:

• ich erzähle meinem Freund ... = I tell my friend ...

This is a very common pattern with erzählen.

Because von is a preposition that always takes the dative case.

So:

• mein Tag would not work after von
• mein Tag becomes meinem Tag in the dative singular

That gives:

• von meinem Tag

In this sentence, von meinem Tag means about my day.

Because erzählen can be used in different patterns, and here German expresses the topic with von + dative.

So:

• jemandem etwas erzählen = to tell someone something
• jemandem von etwas erzählen = to tell someone about something

In your sentence:

• meinem Freund = the person being told
• von meinem Tag = the topic

That is why the sentence means I tell my friend about my day, not I tell my friend my day.

Because both are masculine singular nouns in the dative case.

• der Freund → dem Freund
• der Tag → dem Tag

And the possessive mein- must match that case and gender, so it becomes meinem in both places:

• meinem Freund
• meinem Tag

So even though the meanings are different, the grammar is the same: both are masculine singular dative forms.

Both can be possible, but they are not always used in exactly the same way.

• von meinem Tag erzählen is a very natural, common way to say tell someone about my day
• über meinen Tag sprechen/reden is also very common with verbs like sprechen or reden

You may also hear über meinen Tag erzählen, and many speakers would understand it, but von meinem Tag erzählen is especially idiomatic with erzählen.

So for learners, the safest pattern is:

• jemandem von etwas erzählen

There is no separate article because the possessive word meinem is already doing that job.

In German, possessives like:

• mein
• dein
• sein
• ihr

behave a lot like articles and take endings themselves.

So instead of:

• dem Freund
• dem Tag

you get:

• meinem Freund
• meinem Tag

So the possessive replaces the article rather than appearing next to it.

Yes. German word order is flexible, but the finite verb still has to stay in second position in a main clause.

For example, these are possible:

• Beim Abendessen erzähle ich meinem Freund von meinem Tag.
• Ich erzähle meinem Freund beim Abendessen von meinem Tag.
• Meinem Freund erzähle ich beim Abendessen von meinem Tag.

The meaning stays similar, but the emphasis changes:

• Beim Abendessen first emphasizes the time/context
• Ich first is more neutral
• Meinem Freund first emphasizes who I’m telling

So the original sentence is perfectly normal, but not the only possible order.

Erzählen means to tell or to narrate. It is a regular verb.

The form erzähle is:

• 1st person singular
• present tense

So:

• ich erzähle = I tell / I am telling

Its basic infinitive is erzählen.

In this sentence, it works with:

• a dative object: meinem Freund
• and a von + dative phrase: von meinem Tag

That combination is very common and useful to learn as a pattern.