Examine the verbs in the following sentences.
a. David sings ballads.
b. My aunt sent me a present.
c. *Jim loves.
d. *Him gave me a hand.

1. How many NPs do the verbs in the acceptable sentences take?
Transitive verbs take at least two dependents. That's why c. is unacceptable. Let's take a good look at the acceptable sentences a. and b. first. In a. David, which is the subject of the sentence, has the agentive case (a more common version for you is a doer), and ballads, which is the object of the sentence, has the 'accusative case' (it receives the action).

2. Why is d. unacceptable?
The pronoun forms that have agentive case are 'I, you, he, she, it, we, they', whereas the forms that have accusative case are 'me, you, him, her, it, us, and them'. Therefore, c. is not acceptable since it takes the form of accusative case instead of the agentive case.

Now let's examine the types of transitive verbs by looking at the following examples:

e. Sarah speaks three languages.
f. Her mother makes the bread every friday afternoon.
g. The old man gave the children some money.
h. The detectives asked Sandra lots of questions.

3. How are sentences e. and f. different from sentences g. and h.?
The verbs in sentences e. and f. take two dependents (the dependent that precedes it has the agentive case and the one that follows it has the accusative case). The verbs that behave this way are called '(mono)transitive verbs'.
The verbs in sentences g. and h., however, take three dependents (the dependent that precedes it also has the agentive case and the other two dependents both have the accusative cases).
The verbs that take three dependents are called 'ditransitive verbs'.

4. Which is direct object and which is the indirect object of the ditransitive verbs?

i. The attorney showed the jury the photographs.

There are two arguments to be discussed here.
Early linguistic work proposed that the first noun after the ditransitive verb is the direct object. The proof given was 'passivization'. That is, the first noun is easier to passivize.

j. The jury were shown the photographs.
k. *The photographs were shown the jury.

Recent work, however, argues the second noun after the ditransitive verb is the direct object, and the first noun after the ditransitive verb the indirect object. This approach consider the second noun after the ditransitive verb is similar to the second dependent, which has accusative case, of a monotransitive verb. From this perspective, linguists propose that the second noun after the ditransitive verb should behave the same way the object of a monotransitive verb does. The following examples shall clarify this point:

l. I bought my niece a necklace of pearl on her wedding.
m. I bought a necklace of pearl.

n. Which necklace did you buy your niece?
o. *Which niece did you buy a necklace of pearl?

You may want to take a look at sentences j. and k. again; now we've got the following sentences:

p. The attorney showed the jury the photographs.
q. Which photographs did the attorney showed the jury?
r. *Which jury did the attorney showed the photographs?

Moreover, in sentences such as the ones given below, it's also the second noun after the ditransitive verb that behave the same way an object of a monotransitive verb does.