As an assistant, you will probably be instructed to do this more than once. Numerous times I've shown up to work, only to find out that I had a new class that started in 15 minutes and I had nothing prepared. So I follow this plan to determine the students' speaking and grammar abilities. I know it's difficult to get students to speak for a full 55 minutes, but as long as you change the subject often and give plenty of examples, you'll survive.
1. Bring index cards or small pieces of paper so students can write their names on them. Students give these cards back to assistant. Each time assistant wants to call on a student, s/he chooses a name at random. Students introduce themselves; assistant introduces him/herself.
Students think of other questions to ask someone they meet for the first time. Assistant writes all of their suggestions on the board. Make sure to include questions that are usually translated wrong: How long have you been in France? (instead of How many times are you in France?) or Where were you born? (instead of Where are you born?) Assistant asks some of these questions to the students.
2. Write the question "What do you like to do?" on the board. Elicit several sample responses from students using the forms:
I like to + verb / I like verb+ing / I like + noun
Continue with the negatives:
I don't like to + verb / I don't like verb+ing / I don't like + noun
You can also introduce similar phrases for expressing likes/dislikes: love, enjoy, fond of, hate, detest, etc.
3. Write "What are you going to do this weekend?" and "What will you do this weekend?" on the board. Elicit responses using the forms:
I'm going to + verb / I will + verb
And continue with their negative forms: I'm NOT going to, I will NOT, etc.
4. Write "What did you do last weekend?" on the board and elicit responses using several irregular verbs in the preterit.
I went... / I did... / I had...
And continue with their negative forms: I DIDN'T go, I DIDN'T do, I DIDN'T have, etc.
5. Compare the two groups of verbs in English with respect to word order in negative sentences, i.e. verbs that place NOT after, and verbs that require DO/DOES NOT or DID NOT before. Write sample verbs on board and ask which group they belong to. Include most modal verbs. [You can also include word order in questions, since the two groups of verbs are the same, i.e. invert subject and verb or use DO/DOES/DID as an auxiliary + subject + infinitive.]
6. Continue with modal verbs and have students create sample sentences for them. Introduce other similar phrases that can replace modal verbs. Then students must put them into the past tense.
I have to go / I had to go = I must go
I can go / I could go = I am able to go / I was able to go
I will go / I would go
* Remind them that must can only be used in the present tense!
Continue with putting the present and past modals into negative and review pronunciation of contractions.
I have to go - I don't have to go I had to go - I didn't have to go
I can go - I can't go I could go - I couldn't go
I will go - I won't go I would go - I wouldn't go
7. Choose students at random to change the following sentences (do not write the sentences on the board - see if the students can do this orally):
Present to Past
a. I eat too much chocolate.
b. She buys groceries every Monday.
c. We leave for England at 2 p.m.
d. They drive all day and all night.
e. I find the boy very rude.
f. He teaches history and geography.
Positive to Negative
a. I'm going to school soon.
b. He wears that coat all the time.
c. We made a cake for dessert.
d. She understood what I said.
e. Laura has got two brothers.
f. I have seen him today.
Present/Past to Present Perfect
a. He is very sick and tired.
b. I did all of my homework.
c. She went to see the doctor.
d. We speak to the teacher today.
e. I want to leave for a while.
f. They eat the entire pizza.