Lähettäkää lisätieto sähköpostitse.

Lähettäkää is the imperative form of the verb lähettää, which means to send.

So in this sentence, lähettäkää means send!

More specifically, it is the 2nd person plural imperative, which is used:

• when speaking to more than one person, or
• as a polite form when addressing one person in a formal context.

So the tone is like:

• Send the additional information by email.
• or more politely, Please send the additional information by email.

The ending -kaa / -kää is the normal ending for the 2nd person plural imperative in Finnish.

Because this verb contains front vowels like ä, Finnish vowel harmony requires -kää rather than -kaa.

So very roughly:

• lähettää = to send
• imperative stem + -kää → lähettäkää

You do not need to think of this as a word-for-word translation into English. It is simply the standard Finnish way to say send! to several people or politely.

Yes. Grammatically, lähettäkää is plural.

But Finnish often uses the plural you form as a polite way to address one person, similar to formal you in some other European languages.

So this sentence could be aimed at:

• several people: Please all of you send...
• one person formally: Please send...

In signs, instructions, customer-service language, and official writing, this kind of form is very common.

Lisätieto is a compound word:

• lisä = extra, additional
• tieto = information, piece of information, knowledge

Together, lisätieto means additional information.

A useful thing to know is that Finnish often uses the singular where English might also use an uncountable expression like additional information. So even though English does not say an additional information, Finnish can still use the singular noun naturally here.

Because Finnish can use the singular here in a general or collective sense.

So lisätieto means something like:

• additional information
• further information

If the speaker wanted to emphasize several separate items, details, or documents, lisätiedot could also be possible in some contexts. But in an instruction like this, lisätieto sounds natural and standard.

In other words:

• lisätieto = additional information, as a general thing
• lisätiedot = additional details / additional pieces of information

This is a very common learner question, because Finnish object forms depend on grammar and meaning.

Here, lisätieto is the object of lähettäkää. In an affirmative imperative, a total object in the singular usually appears in the nominative form, which is why we get lisätieto.

So:

• Lähettäkää lisätieto = Send the additional information
  Here the idea is a complete action with a complete object.

Why not the other forms?

• lisätiedon: this form is not used here because imperative clauses do not normally take the singular total object in the genitive form.
• lisätietoa: this would be the partitive, which would suggest something more like some additional information or an incomplete/indefinite amount. In some contexts that could be possible, but it is not the most natural choice for this standard instruction.

So in this sentence, lisätieto is the expected form.

Sähköpostitse means by email or via email.

It comes from sähköposti (email) plus the ending -itse, which creates an adverb-like expression meaning by means of / via.

So:

• sähköposti = email
• sähköpostitse = by email, via email

This form is especially common in:

• instructions
• formal writing
• official language
• notices and requests

It sounds slightly more formal or fixed than some everyday alternatives.

Yes, often you can.

Both can mean by email, but there is a slight difference in style:

• sähköpostitse = a bit more formal, official, instructional
• sähköpostilla = more everyday and conversational

So these are both understandable:

• Lähettäkää lisätieto sähköpostitse.
• Lähettäkää lisätieto sähköpostilla.

In a notice, website instruction, or formal request, sähköpostitse is especially natural.

Because this is a command or instruction, and in Finnish the verb very often comes first in that kind of sentence.

So:

• Lähettäkää lisätieto sähköpostitse. = neutral, natural instruction

Putting something else first is possible, but it would usually add emphasis or sound less neutral:

• Sähköpostitse lähettäkää lisätieto. = marked emphasis on by email
• Lisätieto lähettäkää sähköpostitse. = also marked, less neutral

For a normal instruction, verb-first is the most natural choice.

Finnish has no articles, so there is no separate word corresponding exactly to the or a/an.

That means Finnish often leaves things more open, and English decides from context whether to translate with:

• the
• a/an
• no article at all

Also, Finnish often does not need a separate word for please in instructions. The politeness can come from:

• the plural/polite imperative form lähettäkää
• the overall context
• the style of the message

So Lähettäkää lisätieto sähköpostitse can naturally mean:

• Send the additional information by email.
• Please send additional information by email.

depending on context.