Choose whether the verb form is a Gerund or a Present Participle:
They are building a bridge across the river.
a)Are building- present participle
b)Building- present participle
c)Building- gerund
d)Are- gerund

Hint: A gerund is a continuous form or the ‘ing’ form of the verb that has the same function as a noun. A present participle is also in the ‘ing’ form of the verb but it functions as an adjective or a verb.

Complete step by step answer:

As given in the hint, both gerunds and present participles end in ‘ing form’ but they have different functions.
For example (1)- I like singing. Here, singing is the gerund and it functions as a noun.
Example (2)- This is an amazing book. Here, amazing is the present participle and it functions as an adjective.
Now, let’s look at the given options:
Are building- This option shows the correct form of the verb, as we know a present participle functions as a verb in the form of are + building. Hence, it is the correct option.
Building- This option is inappropriate, as it cannot function without a form of being. Hence, it is an incorrect option.
Building- The building does not function as a noun in the given sentence. Hence, it is an incorrect option.
Are- Are is not a gerund, it is an auxiliary verb or a helping verb. Hence, it is an incorrect option.
The correct answer is Option ‘a’.

Note: You can identify a gerund and a present participle by noticing the ‘ing word’. If the sentence begins with an ‘ing word’, it would be a gerund. But if a sentence has a form of be + ‘ing word’, it would be a present participle.