Where things are and where people go
Describe locations, positions and destinations.
To say where something is, use อยู่ followed by a location. A word such as ใน, inside, or หลัง, behind, can specify the position. If the sentence already has an action verb, a location phrase can follow that action without อยู่: ผมรอตรงประตู means I'm waiting right by the door.
A location word isn't always necessary: อยู่บ้าน means to be at home.
Where exactly is the toilet?
ตรงไหน asks for a more specific spot; ที่ไหน is a general where.
The toilet is behind the shop.
The book is on the table.
The cat is under the bed.
The restaurant is next to the bank.
Location words
English in and at don't map to Thai words one for one. ใน specifically places something inside: ในกระเป๋า, in the bag. ที่ names a location more generally. กินที่ร้าน says where you eat; กินในร้าน specifically puts you inside the restaurant rather than outside it.
A destination can follow ไป, go, directly: ไปตลาด, go to the market; ไปโรงแรม, go to the hotel; ไปสนามบิน, go to the airport. A location word can be added when it helps specify the destination, but isn't required in these basic phrases.
I'm eating at a restaurant.
กินข้าว literally means eat rice, but also commonly means have a meal.
The money is in the bag.
I'm waiting right by the door.
ตรง points to the specific spot by the door.
I'll go to the market.
The destination follows ไป without a separate word for to.
Some expressions already identify a place. ที่นี่ and ตรงนี้ mean here; ที่นั่น and ตรงนั้น mean there. Forms with ตรง point more directly to a spot. ข้างนอก, outside, and ข้างใน, inside, can also serve as location expressions without adding another preposition.
Common mistake
Don't automatically add a Thai word for every English to, in or at. ไปตลาด is a complete destination phrase. In a location phrase, choose ใน when inside matters and ที่ when you simply mean at that place.
Practise
Kai says
Thai verbs don't change their endings for past and future. Context and words such as จะ, a future marker, tell you when something happens.