Old filets of fish will be slimy and have an overly fishy smell. When frozen filets are poorly stored, (refrozen/not flash frozen/stored at improper temperature/too old) they have this texture of eating a sponge that makes me wretch, since the delicate emulsion properties of the flesh have been completely broken down. The moisture can be sucked out and the proteinous membranes chewed on like rough, spongey, dough. If you try to skin them, they sometimes come apart in your hands rather than adhere like tissue from a living organism.
When cooked poorly (burnt to a crisp, overseasoned), it's difficult to tell the difference between fresh and old filets.
There are three major properties of the liquids:
- Fat based (oil, melted butter, bacon drippings, and on) versus water based (milk, juice, water, and so on).
- Flavor - some liquids, juices and to a lesser extent, milk have a distinct flavor that will effect the bread
- Acid - Some liquids, like orange juice, are more acid and affect the pH of the dough
Not every liquid falls neatly into these exact categories. Milk, for example, is about 87% water, 4% fat, and the rest is sugars and proteins. These differences are often insignificant compared to the overall bread recipe.
In general, you can substitute fat-based liquids for other fat based ingredients, and water based liquids for other water based liquids.
Most breads also have the capacity to carry additional fats and sugars (called enrichment), which at its extreme leads to breads like brioche which are very sweet and fatty.
Eggs are a complex ingredient in that the whites are mostly water, with some protein (albumin), and the yolks are about 50% water and 50% fat.
See related:
Are there any general principles of ingredient substitutions? Note that in a bread recipe, with so few ingredients and a deceptively complex chemistry, all of the ingredients have a role to play in the chemistry, so the major factors you are looking at are the overall total ratios of fat, sugar, and water to the flour. Accent ingredients would be add ins like carway seeds in rye bread which don't incorporate into the dough.
In regards to your desire for more olive oil flavor, you almost certainly can substitute olive oil for any fats already in the recipe with comparable results. Most dought can even carry a little more fat, although this will inhibit yeast growth and gluten development, so that you will need more kneading and more proofing time.
You can also brush the crust with olive oil before or after baking.
The very best way to get olive oil flavor into your bread is to dip it in oil when you eat it. This also allows you to get the full flavor of a quality oil, as many nuances would be lost if it was baked into the bread.
Best Answer
You have two fairly different questions here: flavor pairing (what combinations taste good), and prediction of flavors.
Prediction
Eat as diversely as possible. Try things with different ingredients and combinations of ingredients. It's best if it's something you've cooked, so you know exactly what goes into it, but even eating in restaurants helps, especially if you look carefully at menus and at what you're eating. Eventually you'll learn your personal preferences better, and develop intuition for what everything tastes like.
There's really no substitute for this in the end, because everyone has different preferences, so ingredients and combinations that taste good to you might not quite line up with what others like. And besides, it's not really possible to describe flavors meaningfully, so even if someone tells you "you'll love strawberry and vanilla together", you ultimately have to just learn the flavors to be able to predict how it'll taste to you.
Pairing
Specifically on combinations, see this previous canonical question. The summary is that again, this can be pretty personal, but insofar as there are common preferences, it's difficult to generalize. Most useful resources end up being long lists of pairings you might like, just slightly more general than looking for recipes that contain one of the ingredients and seeing what else is in them.