Why is yeast important? Because it converts sugar to alcohol. First, you don't need to add yeast. All grapes and winery equipment are covered with both good and bad native yeast strains that will start your ferment. Most people add yeast to control the ferment.

Grapes are covered in wild yeast and bacteria. Thankfully most of these strains don't like SO₂, so you can do your wine a favor and add some SO₂ to the crushed grapes to inhibit these strains.

The wild yeast that you want to survive can survive in the presence of some SO₂ and not be bothered.

SO₂ can also protect juice from some enzymatic browning. Note that SO₂ has no effect against the browning enzyme Laccase (produced by the Botrytis mold.)

If you live in a warm climate, chances are your grapes develop ripe flavors late enough that the grape's natural acid has been diminished. In this case you should adjust the acid back to what the Winemaker think is the appropriate level.

Like with most things, the earlier you can solve a problem, the better. Adjusting the acid before or during fermentation is better than waiting until after fermentation or ML conversion to do so.

Even if you still need to make an acid add later, it would most likely be a small acid correction, as opposed to a major change in acid and pH.

Brix is a measure of soluble solids, and in grape juice that is mostly sugar. The yeast converts the sugar into alcohol, and the resulting alcohol plays an important role in the character and balance of our wine.

Depending on where you live, grapes are probably ripening with sugars higher than you want, or lower than you want. In these cases it's important to correct the sugar content of the juice or must so that you end up with an alcohol content that will fit your wine stylistically.

In warm climates that means adding water to the must to reduce the sugar content, and in cold climates that means adding sugar to the must to increase the sugar content. Note that you can add either granular sugar or grape juice concentrate to achieve this goal, and depending on where you live one of these options is probably illegal.

If you're dealing with a variety of grapes that tends to raisin (such as Zinfandel) or a vintage with a higher percentage of raisins, you should wait 2-3 days for the must to 'soak up' the sugar out of these raisins before correcting the Brix.

Nutrients are required by yeast to ferment sugar into alcohol. The primary yeast nutrient is ammonia (NH₃), added as Diammonium Phosphate (DAP).

There are lots of other amino acids, minerals and other so-called micro-nutrients required by yeast that may or may not be present in the juice. In this addition calculator they are referred to as "Generic", since you're adding a rate of the overall mixture, not a particular component of the mixture.

If yeast don't have enough of a particular nutrient, the fermentation will likely become stuck, so you want to make sure you give the yeast the food they need to complete the fermentation. Adding excess nutrient, however, can leave behind enough nutrients to encourage unwanted microbial growth.

A good rule of thumb is higher nutrient levels for higher brix levels, and also higher nutrient levels for moldy fruit.

When a fermentation gets stuck leaving residual sugar is an invitation for spoilage organisms and a headache to have in the cellar, so it's better to referment and get the wine dry than deal with the alternative.

It's best to understand why the fermentation stuck to know the best route to referment. If you're not sure why, take a sample to an outside lab and ask for their advice.