A group of very common words decline in the third declension for their masculine and neuter and the first declension for their feminine. They are small words, and several of them carry a great deal of weight.
The pattern is worth naming once: 3-1-3. Masculine third, feminine first, neuter third. You will meet it again in the participle, so it is worth getting comfortable here.
πᾶς: the forms
πᾶς, πᾶσα, πᾶν — "all", "every" — occurs 1,244 times, which makes it one of the commonest words in the New Testament.
The masculine and neuter run on the stem παντ-: παντός, παντί, πάντα, πάντες, πάντων. The feminine is πᾶσα, πάσης, πάσῃ, πᾶσαν, on the first declension pattern.
The dative plural is worth a look, because it is the collision from chapter 17 happening again: παντ- + σι gives πᾶσι(ν), with the ντ vanishing and the α compensating by lengthening.
How πᾶς reads
Position changes it, and the difference matters in argument.
With no article, singular: usually "every".
With no article, plural: "all" — πάντας ἀνθρώπους, "all people".
With the article, in predicate position: "the whole".
With the article, in attributive position: "the entire", and it is rare.
πᾶσα γραφὴ θεόπνευστος
"Every scripture is God-breathed." No article, singular — so "every" is the likely reading, though "all scripture", as the KJV has it, is also defended.
καὶ ἰδοὺ πᾶσα ἡ πόλις ἐξῆλθεν εἰς ὑπάντησιν τῷ Ἰησοῦ
"And behold, the whole city went out to meet Jesus." Article present, πᾶσα outside it — so "the whole", not "every".
ἵνα πᾶς ὁ πιστεύων εἰς αὐτὸν μὴ ἀπόληται
"So that everyone who believes in him should not perish." πᾶς with an articular participle — "everyone who…" — is one of the commonest constructions in John, and you will meet the participle half of it in chapter 21.
εἷς, οὐδείς and μηδείς
εἷς, μία, ἕν is "one" (342 occurrences), and it is 3-1-3 like πᾶς: ἑνός, ἑνί, ἕνα in the masculine.
Two words are built straight onto it. οὐδείς, οὐδεμία, οὐδέν is οὐδέ + εἷς, "not even one" — so, "no one", "nothing" (232). μηδείς is the same word with μή in front, used where μή would be the negative — outside the indicative (90).
Οὐδεὶς δύναται δυσὶ κυρίοις δουλεύειν
"No one can serve two masters." The rest of that verse gives you τὸν ἕνα and ἑνός as well, so it is worth reading whole.
Note δυσί: the dative of δύο, and one of only a handful of forms that word has.
πολύς and μέγας
Two adjectives that look irregular and are not, quite. Both have a short nominative and accusative singular, and then behave normally on a longer stem.
πολύς, πολλή, πολύ, "much, many" (415) — but the genitive is πολλοῦ and the plural πολλοί, πολλῶν, πολλοῖς, all on πολλ-.
μέγας, μεγάλη, μέγα, "great" (240) — genitive μεγάλου, plural μεγάλοι, all on μεγαλ-.
So each has three forms that look odd and a full set that does not. Learn the odd ones — πολύς, πολύν, πολύ, μέγας, μέγαν, μέγα — and read the rest as ordinary adjectives.
Comparison
Greek adjectives compare much as English ones do. The regular endings are -τερος for the comparative and -τατος for the superlative, added to the stem: ἰσχυρός, "strong", gives ἰσχυρότερος, "stronger".
The common ones are irregular, as they are in English: μέγας gives μείζων, "greater"; πολύς gives πλεῖον, "more"; ἀγαθός gives κρείττων, "better"; κακός gives χείρων, "worse".
Greek says "than" in two ways. Either the thing compared goes into the genitive:
οὐκ ἔστιν δοῦλος μείζων τοῦ κυρίου αὐτοῦ
"A slave is not greater than his master" — τοῦ κυρίου is a genitive of comparison, and English has to supply the "than". Or the particle ἤ is used, with both sides in the same case:
ἠγάπησαν οἱ ἄνθρωποι μᾶλλον τὸ σκότος ἢ τὸ φῶς
"People loved the darkness rather than the light."
When the comparative is doing superlative work
This is the part worth carrying into a sermon, and the part most often got wrong.
Greek comparatives frequently carry a superlative sense. The New Testament has 264 comparative adjectives and only 42 superlatives, and the gap is made up by comparatives doing the work.
μείζων δὲ τούτων ἡ ἀγάπη
Literally "and greater of these is love" — and almost every English translation gives "the greatest of these is love", rightly. Darby keeps the comparative, "the greater of these", which is the one place you can see the switch being declined. The comparative form is doing superlative duty. Note that the genitive is not what tells you so: τούτων is the same shape as the genitive of comparison in John 13:16 above, and "greater than these" is a possible reading of the words on their own. What settles it is that Paul has just named three things and is ranking them.
It runs the other way too. A superlative is often merely intensive — "very", rather than "most". So neither form settles the sense on its own; the sentence does.
Numbers worth knowing
Of the numerals you count on, only the first four decline. From five to a hundred they are fixed forms, which makes them among the easiest words in the language. The hundreds and thousands are ordinary adjectives again and decline like any other — διακοσίων δηναρίων, χίλια ἔτη — but they are rare enough to meet as they come.
εἷς, μία, ἕν (one) · δύο (two) · τρεῖς, τρία (three) · τέσσαρες, τέσσαρα (four) — then πέντε, ἕξ, ἑπτά, ὀκτώ, ἐννέα, δέκα, and on.
Two are worth knowing for their own sake. δώδεκα, twelve, occurs 75 times — most often of the disciples, but also of the tribes, the baskets and the thrones. And ἑπτά, seven, structures a great deal of Revelation.