I recently had a conversation with one of our students about the meaning of a “declension.” It’s a rather abstract idea. It can be really confusing and one of the earliest roadblocks to a knowledge of Greek.
Here are a couple definitions of ‘declension,’ and then an illustration…
Sec. 6.7. We discussed in sec. 5.7 how there are different patterns that English nouns follow in forming their plural. Some add “s,” others add “es,” while others change the vowel in the stem of the word (e.g., “men”). The pattern a word follows does not affect its meaning, only its form. “Children” and “childs” would mean the same thing, if the latter were actually a word. In Gk there are basically three inflectional patterns used to create the different case endings. Each of these patterns is called a “declension.” (Mounce, Basics of Biblical Greek).
declension. n. A grouping of similarly inflected words —nouns, pronouns, adjectives or participles—according to a discernible pattern. Often grammars speak of three such patterns in Greek, termed first, second and third declension (Pocket Dictionary for the Study of NT Greek)
The way I tried to illustrate this several weeks ago was like different paths leading to the same place. Let me try it again. Think of it as a word wanting to travel from singular to plural. In English, most words take the “add s” road:

However, not all words can take this road. They must find a different road to get to the plural:

These two roads are two different “declensions,” i.e. two different pathways a word might take to change its form (or inflection) and move from singular to plural.
In the same way, in Greek, some words travel from singular to plural by adding an iota. For example the word kurios (lord):

Other words cannot take this path. They must find a different road to get to the plural. They might add an alpha instead. For example, the word ergon (work):

The question, then becomes, how do you know which path a Greek word should take? The answer is the stem endings. Stems ending in alpha or eta (usually feminine) are in the first declension, and follow the first declension “path” from singular to plural. Stems ending in “omicron” (usually masc. and neuter) are in the second declension and follow the second declension “path” from singular to plural. Stems ending in a consonant (which we don’t see ‘til chapter 10) are in the third declension, and follow an alternative “path.”
With all that said, here’s the first and second declension paradigm again:

I hope that makes a little more sense. Please let me know if you have questions. It’s important to get at least a basic grasp of this before we get deeper into Greek.
Thanks,
Pastor Stephen