The will be used before Tea because – Here, we are talking about a particular tea, followed by a Relative Pronoun (which). The sentence can also be written as –

The tea I am drinking is hot (tea-definite) Look at the examples given below : The flowers in her garden are beautiful. (Definite – the ones in her garden)

The flowers (that) are in her  

↓

Rel.pro.

garden are beautiful.

The dog in the backyard is very cute. (Definite-the one in the backyard)

The dog (which) is in the backyard is very cute.

↓

Rel.pro.

Hence, the tea is the right usage.

have seen will replace saw because – Since (Prep.) : from a time in the past until a later past time, or until now, used with the Present Perfect or Past Perfect Tense. Look at the examples given below : She has been off work since Tuesday. He has been working in a bank since leaving school. Hence, I have seen him is the right usage.

There are no error

was will replace are because– when a Plural Noun is placed with a real Singular Subject (one, each, etc.) with of, the Verb will be Singular. Look at the examples given below : One of the boys is ill. Each of the girls is clever. Hence, was shot dead is the right usage.

contains will replace contain because – The fifth and final act (Main Subject) is in Singular. Hence, the Verb will also be in Singular as a Verb agrees with its Subject in Number. Singular Subject ® Singular Verb (Main Verb + S) Plural Subject ® Plural Verb (Main Verb without S) Look at the example given below: The great poet and scholar is dead. Here, scholar and poet refer to the same man. So, the Subject is in Singular and the Verb is also in Singular.

to enter will replace entering because – The to-Infinitive (Main Verb) : functions as a Noun, as Adjective, or an Adverb. In this sentence, permitted to enter, to enter is the direct object, the Noun that receives the action of the Verb (Permitted). To enter refers to a thing being done, or, desired to be done Hence, to enter the park is the right usage.

away will not be used because– to pass away (Phr. V.) : to die; end to pass time : to spend time, especially when you are bored or waiting for something/someone to pass (Verb) means to move past Look at the examples given below : The road was so narrow that cars were unable to pass. They passed the time until dinner talking and playing cards. My aunt passed away last month. The years of famine passed away and were followed by years of prosperity. Hence, to pass the time is the right usage.

had I will replace I had because – When a negative Adverb/Adverbial Expression (no sooner, hardly/barely/scarcely), is used at the beginning of a clause; it is followed by Aux. Verb + Subject.

Look at the examples given below :

No sooner had he finished dinner

                   ↓    ↓

            Aux.V. Subject

than he started feeling sick.

Hardly had she arrived, when problems started.

              ↓    ↓

        Aux. V. Subject

Hence, had I spoken is the right usage