When transition questions are discussed in regard to SAT Writing/ACT English, they tend to be covered in two main forms. 

The first way involves a transition placed after a comma in the middle of a sentence. 

Version #1: The Spanish conquest of the Aztecs in 1519 brought the fragrant vanilla flower—and its companion, cacao—to Europe. Vanilla was cultivated in botanical gardens in France and England, but growers were unable to collect its glorious seeds.  

Here, the transition but is placed halfway through a sentence and used to connect the two halves of that sentence. 

The second way involves transitions between sentences (or occasionally two parts of a sentence separated by a semicolon).

Version #2: The Spanish conquest of the Aztecs in 1519 brought the fragrant vanilla flower—and its companion, cacao—to Europe. Vanilla was cultivated in botanical gardens in France and England. However, (or: England; however,) growers were unable to collect its glorious seeds.

In the sentence above, the transition however is used at the beginning of a sentence to indicate the contrasting relationship between that sentence and the previous sentence. 

So far pretty straightforward, right? 

There is, however, another way in which the second version of the sentence can be written. 

Version #3: The Spanish conquest of the Aztecs in 1519 brought the fragrant vanilla flower—and its companion, cacao—to Europe. Vanilla was cultivated in botanical gardens in France and England. Growers were unable, however, to collect its glorious seeds.

Although this version of the sentence places the transition halfway through the last sentence, it is actually identical in meaning to version #2. The transition has simply been moved from the beginning of the sentence to the middle. Its purpose is still to convey the relationship between the last sentence and the previous sentence; it does not connect the two halves of the sentence in which it appears.

When many test-takers see this type of question, however, they do not realize that they need to look back at the previous sentence to determine the relationship. As a result, they either plug in each answer choice into the sentence and then become confused when they are unable to determine the answer or, worse, sit and stare at the question. 

So how do you tell when an underlined transition in the middle of a sentence is being used to connect two halves of a sentence, or to connect one sentence to the previous sentence? 

The shortcut is to look at the commas around the transition. 

Transitions that come after a single comma are connecting two halves of a sentence. 

In contrast, transitions that are surround by two commas — one before, one after — are connecting a sentence to the previous sentence. 

And if you’d like the technical explanation, here goes:

Transitions that follow a comma only are called coordinating conjunctions, also known as FANBOYS conjunctions (For, And, Nor, But, Or, Yet, So). These conjunctions serve to join two independent clauses (complete sentences) into a compound sentence, as in version #1 above. In formal English, these conjunctions should not be used to begin a sentence — that is, they should not follow a period or semicolon.

Transitions used to begin sentences are known as conjunctive adverbs. Common examples include however, therefore, furthermore, indeed, and similarly. The purpose of these transitions is to indicate the relationship between one sentence and the previous sentence.

Usually, these transitions appear at the beginning of a sentence or clause — that is, after a period or semicolon. Sometimes, however, writers move them to the middle of a sentence for the sake of stylistic variety, as in version #3. In such cases, these transitions are used non-essentially — that is, they are placed in between two commas.

To reiterate: only the placement of the transition changes, not the purpose. To determine whether the transition is correct, you must look back at the previous sentence and determine its relationship (continue, contrast, cause-and-effect) with the sentence in which the underlined transition appears. 

Now let’s look at a test-style example:

Despite their iconic look and their important role in Dutch history, wooden clogs are now mostly made

for tourists rather than for everyday wear. The 300,000 pair of shoes made every year are, however,

sold mostly to foreign buyers.

A) NO CHANGE
B) ironically 
C) in fact 
D) meanwhile

The fact that the underlined transition is surrounded by commas indicates that it is necessary to look at the previous sentence to determine the relationship. 

What does the previous sentence tell us? That clogs are mostly made for tourists. 

What does the sentence in which the transition appears indicates? That most of the clogs made each year are sold to foreign buyers (i.e. tourists). 

Are those similar ideas or different ideas? Similar. 

So the correct transition must convey a similar relationship. The only option that fits is C), in fact, because that phrase is used to emphasize a preceding statement.