Første indtryk betyder meget, og et godt indtryk hjælper også, når De møder nye kunder.

De is the formal/polite word for you in Danish. It is often capitalized in older or more formal writing, especially in business language.

So når De møder nye kunder means when you meet new customers, but in a polite form.

In modern everyday Danish, people usually say du instead: ... når du møder nye kunder.

Here første indtryk is being used in a general sense, like English first impressions in a broad statement such as First impressions matter.

Danish often leaves out the article in this kind of general statement. If you were talking about one specific first impression, you would normally use an article, for example:

• et første indtryk = a first impression
• det første indtryk = the first impression

Formally, indtryk can be both singular and plural in the indefinite form.

• et indtryk = an impression
• flere indtryk = several impressions

So the word form itself does not tell you the number. You understand it from context.

In this sentence, Første indtryk betyder meget is best understood as a general idea, and in natural English it is often translated as First impressions mean a lot.

Because indtryk is a neuter noun in Danish.

That means:

• the article is et
• the adjective usually gets -t in the singular indefinite neuter form

So:

• et godt indtryk = correct
• en god kunde = correct, because kunde is a common-gender noun

This is a very common Danish pattern:

• en god dag
• et godt råd

Because kunder is plural, and adjectives before plural nouns usually take -e.

So:

• en ny kunde = a new customer
• nye kunder = new customers

This -e form is also used in several other situations, such as before definite nouns.

Because meget here means a lot or very much, and it is modifying the verb betyder.

So:

• betyder meget = means a lot / matters a lot

Mange is used with countable plural nouns:

• mange kunder = many customers

So the difference is:

• meget = much / a lot
• mange = many

In a normal Danish main clause, the finite verb usually comes in second position. This is the famous V2 word order.

So in: et godt indtryk hjælper også

the verb hjælper comes early, and også comes after it.

This placement is very typical in Danish:

• Han kommer også
• Jeg forstår det også

If you move også, the emphasis can change, but the version in your sentence is the most neutral and natural one.

Because når introduces a subordinate clause, and subordinate clauses have a different word order from main clauses.

After når, the subject normally comes before the verb:

• når De møder nye kunder = correct

Compare that with a main clause question:

• Møder De nye kunder? = Do you meet new customers?

So the sentence uses normal subordinate-clause order: når + subject + verb

Danish often uses the present tense for things that happen in the future, especially after words like når (when).

So:

• når De møder nye kunder

can mean:

• when you meet new customers
• when you are meeting new customers
• when you will meet new customers

This is very natural in Danish and is similar to English, which also often uses the present after when:

• When you meet new customers, ... not usually
• When you will meet new customers, ...

Hjælper is the present tense of hjælpe (to help).

In this sentence, et godt indtryk hjælper også means something like:

• a good impression also helps
• making a good impression also helps

So the idea is that a good impression is useful or beneficial, especially when meeting new customers. Danish often allows this kind of wording where the noun phrase itself acts as the thing that helps.