Q:  What is the difference between ‘I used to play tennis’ and ‘I am used to playing tennis’?  (Johana, Portugal)

That’s a very interesting question, Johana, and one that many students of English struggle with.  Both sentences are grammatically correct, but don’t worry, as there is a very simple explanation.

Let’s take the first sentence –
I used to play tennis.

The explanation here is that the habit, or routine, is in the past and totally finished.  We could say ‘When I was 13, I used to play tennis every weekend’. Here, we can see that is was in the past (I am not 13 years old now), it was a habit (every weekend) and it is totally finished.  Note that the verb form here is used + to + infinitive.
When forming negatives, the form is not + use + to + infinitive as in this example;
I didn’t use to play tennis.
When making questions, the form is did + subject + use + to + infinitive as in this example;
Did you use to play tennis?

Please remember we cannot use this structure in present or future tenses, when talking about specific times in the past or saying how long it took or how many times – for example;
I worked very hard last month. (NOT I used to work very hard last month.)
I lived in London for four years. (NOT I used to live in London for four years)
I went to Paris twice when I was young.  (NOT I used to go to Paris twice when I was young.)

Now, how about that second sentence?

I am used to playing tennis.

The first thing to notice here is the presence of the verb ‘to be’, followed by an ‘-ing’ form of the verb – form of to be + used + to + verb + ing and in negative; form of to be + not + used + to + verb + ing.
The meaning here is that if someone is used to doing something, then it is familiar to them; he or she has experienced it so much that it is no longer strange or new.  For example;
I’ve lived in London for five years, so I’m used to hearing the noise.
I’ve only lived in London for two days, so I am not used to being cold..

In question forms, the structure is form of to be + subject + used + to + verb + ing with this example;
Are you used to living in the UK?

Note that instead of having a verb + ing form, you can use just a noun, for example;
I’ve lived in London for five years, so I’m used to the noise.

We can also use the verbs get and become before used to + verb +ing /noun to emphasise the process, for example;
It took a long time to get used to the weather.

To practise, see if you can work out which form, used to + infinitive or used to + verb + ing should go in the spaces with the verb in brackets.

1 – It took us ages to get ……………………(live) in a flat.
2 – Lots of trains used …………………(stop) here, but not anymore.
3 – I’ll have water.  I’m not used …………………..(drink) alcohol.
4 – When I was a child, I used …………………(dream) of being a teacher.