Διαβάστε οτιδήποτε σας στείλουν πριν το υπογράψετε.

They are 2nd-person plural forms, but in Greek that can mean either:

• more than one person
• one person in a polite/formal way

So this sentence could be addressed to:

• a group: Read anything they send you before you sign it
• one person formally: Read anything they send you before you sign it

The pronoun σας works the same way: it can mean you all or polite singular you.

Διαβάστε is the aorist imperative of διαβάζω. In simple terms, it presents the action as one complete act:

• Διαβάστε = Read it
• διαβάζετε = more like be reading / read regularly / keep reading

In instructions and warnings, Greek often uses the aorist imperative when the speaker means do this once, fully, before the next step. That fits this sentence well: first read it, then sign it.

οτιδήποτε means anything at all or whatever.

So:

• οτιδήποτε σας στείλουν = anything they send you / whatever they send you

It is very indefinite and broad. The idea is:

• no matter what it is
• whatever document or text they send

It is stronger than κάτι, which just means something.

Here σας means to you.

So:

• σας στείλουν = they send you
• literally, Greek orders it like to-you send

This is an unstressed object pronoun. In Greek, these little pronouns usually come before the verb.

Also, in this sentence σας is not possessive. It does not mean your here. It means you as the person receiving something.

στείλουν is the 3rd-person plural perfective non-past form of στέλνω. In learner-friendly terms, it is the form Greek uses here for an indefinite/future-like complete action:

• οτιδήποτε σας στείλουν = whatever they send you

Greek often uses this kind of form after words like οτιδήποτε, όποιος, όπου, etc., when the meaning is whoever/whatever/wherever may...

As for they: Greek usually does not need a separate subject pronoun, because the verb ending already tells you the person and number.

• στείλουν already means they send / they may send

So the sentence does not need an extra αυτοί.

το means it.

It refers back to the thing being sent, probably something like:

• a document
• a contract
• a form
• a paper

So:

• το υπογράψετε = sign it

Greek unstressed object pronouns usually come before the verb, unlike English, where it usually comes after:

• Greek: το υπογράψετε
• English: sign it

Yes. After πριν (before), Greek normally uses a subjunctive-type non-past form:

• πριν το υπογράψετε = before you sign it

The form υπογράψετε is perfective, which fits a single completed action such as signing.

A useful point for learners: after πριν, Greek often omits να, even though the clause has subjunctive meaning. So this is perfectly normal Greek.

You may also encounter πριν να..., but πριν το υπογράψετε is very natural and common.

They are close, but not exactly the same.

• ό,τι = whatever / anything that
• οτιδήποτε = anything at all / whatever at all

In many contexts they can be translated similarly, but οτιδήποτε is often a bit broader or more emphatic.

Also, learners often notice the spelling difference:

• ό,τι is written with a comma to distinguish it from ότι = that
• οτιδήποτε is written as one word

In this sentence, οτιδήποτε strongly emphasizes: no matter what they send you, read it first.