prepositions - Prepositional Phrase - confused - at or in

Let's meet in the morning or at the morning.

in the afternoon or at the afternoon.

in the night or at the night.

in the park or at the park.

working in the background or at the background.

Answer

Let's take them one at a time:

Let's meet [in or at] the (morning or afternoon).

This is to meet at a general time in the next (most immediate) morning. "In" is the correct choice. The resolution applies to both 'morning' and 'afternoon,' as each refers to a non-specific time frame.

Let's meet [in or at] the night.

"The" here is where one might trip. For someone proposing a clandestine effort, to say "Let's meet in the night" is arguably plausible if uncommon. More conventionally, however, one would say "Let's meet at night," with no need to use the definite article "the," as each day has only one night.

Let's meet [in or at] the park.

You can make a valid case for either preposition. The park is a specific location when seen on a map, but any location within the park is also "in" the park.

working (in or at) the background

In most contexts, foreground and background are an abstract, relative relationship, not a fixed point, thus making it difficult to rationalize "at." If you have, for example, three layers, the middle is in the foreground relative to the rear-most layer, but in the background relative to the nearest layer. Also, since "working" implies a use of the word "background" in a notional sense of a secondary process or task, not a specific place, I would almost always recommend "in" as the correct choice.