The degree of concreteness or abstractness of a concept is not related to whether its noun is countable or not (see uncountable-noun), and it is the countability of a noun which determines whether it takes a determiner, and whether or not it can be pluralized.
Here, verb is a countable noun, whereas conjugation is an uncountable noun (or mass noun or noncount noun), even though they are both theoretical constructs. A great many purely conceptual ideas are countable— idea itself is countable, as are decision or charism or noumenon.
At the same time, words for substances— stone, butter, fog— are often uncountable, as are words for some classes of tangible objects, like glassware, furniture, or housing.
What is tricky is that there are many situations where a non-count noun can be used as a count noun. This is probably most common, especially in informal situations, where a substance has been packaged or divided into discrete units for consumption.
Person A: Let's order dessert for the table. How many desserts should we get?
Person B: Ice cream is a dessert I definitely love— we should order three ice creams.
Here, it is understood that person B wants to order three servings of ice cream, in whichever way the restaurant serves it.
Another situation involves varieties of something uncountable.
The dessert specials were for chocolate cake, butterscotch ice cream, and rum raisin ice cream. The ice creams were far more popular than the cake; in fact, either ice cream on its own was more popular than the cake.
As you have not provided complete sentences, we have to assume that conjugation is being used in a non-countable way in your original question. But you can speak of examples or categories of conjugation in a countable way as well:
Traditionally, there are four basic Latin conjugations. The third conjugation has a huge variety of patterns, compared to a conjugation like the fourth which is relatively regular.