Acts 1:16

 Ἄνδρες ἀδελφοί, ἔδει πληρωθῆναι τὴν γραφὴν ταύτην, ἣν προεῖπεν τὸ πνεῦμα τὸ ἅγιον διὰ στόματος Δαυὶδ περὶ Ἰούδα, τοῦ γενομένου ὁδηγοῦ τοῖς συλλαβοῦσιν τὸν Ἰησοῦν.

Men, brethren, it was necessary for this Scripture to be fulfilled, which the Holy Spirit spoke beforehand through the mouth of David concerning Judas, the one becoming a guide to the ones arresting Jesus.

The verse begins with a capitalized Ἄνδρες, as a convention for reporting direct speech (Peter is speaking, from verse 15).

Ἄνδρες ἀδελφοί seems to be a somewhat formal term of address, and corresponds to Ἄνδρες Ἀθηναῖοι ('men of Athens') seen in, e.g., Plato and Demosthenes.  And Paul will use Ἄνδρες Ἀθηναῖοι later in Acts (17:22).

The form ἔδει is 3-S, imperfect active indicative, from δεῖ, an impersonal verb - 'it is necessary', 'it is right'.  This form requires an infinitive to complete its meaning, in this case πληρωθῆναι.

Πληρωθῆναι, in turn, is the aorist passive infinitive of πληρόω, 'I fulfill, I complete'.

Προεῖπεν is a hapax legomenon, that is, this verb is used only once in the New Testament.  On the other hand, it is a combination of the preposition πρό, meaning (generally) 'before', and a very common verb: λέγω / εἶπον.  So the basic idea is something like 'I speak before', or even 'I predict'.  

Note that the English 'predict' derives from the Latin preposition prae, 'before', and the Latin verb dicere, 'to say', giving a close correspondence to the Greek προεῖπεν. 

Τὸ πνεῦμα τὸ ἅγιον, if translated word-for-word, emerges as 'the spirit the holy', i.e., the Holy Spirit.  The word order does not follow English idiom, but is common in Greek.

The most confusing part of this verse is, I believe, the final phrase, which follows the name Ἰούδα, and modifies it:

περὶ Ἰούδα, τοῦ γενομένου ὁδηγοῦ τοῖς συλλαβοῦσιν τὸν Ἰησοῦν

The name Ἰούδας refers to several different people in the New Testament, and also to the tribe of Judah.

In this case, from context, Peter is speaking about Judas Iscariot, the apostle.  Ἰούδα is the genitive form of this name, and - as a result - the words τοῦ γενομένου ὁδηγοῦ ('him / the one having become a guide') are also in the genitive.

So Judas is the one who became a guide, or was guiding.  Who?   

English idiom uses the preposition 'to', i.e., Judas was a guide 'to someone'.  Greek just uses the dative case:

τοῖς συλλαβοῦσιν

to the ones having arrested

Συλλαβοῦσιν is the masculine plural dative of the aorist active participle of συλλαμβάνω, which has a variety of meanings, including 'arrest / capture', 'help' (?), and 'get pregnant'.

In this case, it takes the direct object of the person arrested or captured, τὸν Ἰησοῦν.