• if, forasmuch as, that, although, whether
• Conditional particle:
  • if
  • since
• Introducing a direct question:

  λέγων, εἰ τῷ ἑκατονταετεῖ γενήσεται, καὶ εἰ Σάρρα ἐνενήκοντα ἐτῶν οὖσα τέξεται
  saying: "Would one who is 100 and Sarah who is 90 give birth?" (Gen 17:17)

• Introducing an indirect question: whether

  σὺ ἠρώτησας τοὺς παῖδάς σου λέγων Εἰ ἔχετε πατέρα ἢ ἀδελφόν
  You asked your servants, saying whether you had a father or brother (Gen 44:19)

• After verbs of swearing: No, Not, Never (a Hebraism expressing strong negation even after an implied oath)

  ὀμνύει κύριος καθ’ ὑπερηφανίας Ἰακὼβ εἰ ἐπιλησθήσεται εἰς νεῖκος πάντα τὰ ἔργα ὑμῶν.
  The Lord swears by the pride of Jacob: All your deeds shall never be forgotten for argument. (Amos 8:7; also see Gen 14:23; Num 14:28-30; Deut 1:35; Ezek 16:48)

• εἰ ἀπὸ = whether from

  εἰ ἀπὸ σπαρτίου ἕως σφαιρωτῆρος ὑποδήματος λήμψομαι ἀπὸ πάντων τῶν σῶν
  whether from a strap to a sandal thong, I won't take all from you

• εἰ ἄρα = if then, if on the other hand (Mark 11:13; Acts 8:22; 17:27)
• εἰ γάρ = indicates an intense wish, see Job 6:8
• εἴ γε = if indeed, inasmuch as (2Cor 5:3; Col 1:23)
• εἰ δὲ καί = but if, and if, and even if
• εἰ δὲ μήγε = otherwise
• εἰ δὲ μή γε = otherwise (Matt 9:17; Luke 5:37; 10:6; 13:9; Matt 9:17; 2Cor 11:16)
• εἰ δὲ μή = if not, otherwise (Mark 2:22; John 14:11; Rev 2:5, 16)
• εἰ ἐκ = whether from

  δοκιμάζετε τὰ πνεύματα εἰ ἐκ τοῦ θεοῦ ἐστιν
  test the spirits whether they are from God (1John 4:1)

• εἰ ... ἤ = shows several choices, e.g., if this or if that
• εἰ verb ... ἤ verb (in a question) = should I do this or should I do that?
• εἰ καί (εἰ καὶ) = even if, even though, although (1Cor 7:1; 2Cor 4:16; 5:16; 7:8³; 7:12;11:15; 12:11; Heb 6:9; Luke 11:8; 18:4; Mark 14:29; Phil 2:17; 3:12)
• εἰ μὲν γαρ = for if ... then
• εἰ μὲν οὖν = if, then (Acts 19:38; 25:11; Heb 7:11; 8:4)
• εἰ μέντοι = if, however
• εἰ μή = unless, except that, except for, apart from, aside from, with the exception of; if not; but (Gal 1:7)
• εἰ μή ὅτι = except that, unless
• εἰ μήν (εἶ μήν) = surely, certainly
• εἰ μήν = if truly, if surely
• εἰ μήτι = unless indeed, unless perhaps
• εἰ οὖν = if, therefore (Acts 11:17; John 13:14; 18:8; Luke 11:13, 36; 12:26; 16:11; Matt 6:23; 7:11; 22:45; Phe 17)
• εἴ πως (εἰπως) = if perhaps, if somehow, if by some means (Acts 27:12; Phil 3:11; Rom 1:10; 11:14; 2Sam 14:15; 16:12; 1King 20:31; 2King 19:4; Job 20:23; Jer 51:8)
• εἴ τις, εἴ τι = everyone who, everyone whoever, everything that, everything whatever
• ἐκτός εἰ μή = unless, except
• ὅτι εἰ μή = except for
• καὶ εἰ = if only

  καὶ εἰ κατεμείναμεν ... παρὰ τὸν Ἰορδάνην
  if only we had remained ... beyond the Jordan

NT: Gal 3:4; 2Cor 5:3
LXX: _
Apocrypha: _
Apostolic Fathers: _