Jag lägger lerklumpen på drejskivan.

Because lägga and ligga are different verbs in Swedish.

• lägga = to lay / to put something somewhere
  
  • it usually takes an object
• ligga = to lie / to be lying somewhere
  
  • it describes position, not the act of placing

So:

• Jag lägger lerklumpen på drejskivan. = I put / am putting the lump of clay on the wheel.
• Lerklumpen ligger på drejskivan. = The lump of clay is lying on the wheel.

This is similar to older English lay vs lie, though Swedish uses the distinction much more clearly in everyday speech.

lägger is the present tense of lägga.

Basic forms:

• infinitive: lägga = to put
• present: lägger = put / am putting
• past: lade = put
• supine: lagt = put

So Jag lägger ... means:

• I put ...
• I am putting ...

Which English translation fits best depends on context.

Because lerklumpen is the definite singular form of lerklump.

• en lerklump = a lump of clay
• lerklumpen = the lump of clay

In Swedish, the definite article is usually attached to the end of the noun instead of being a separate word like English the.

So:

• en bok = a book
• boken = the book

The same pattern is used here:

• en lerklump
• lerklumpen

For the same reason: it is also a definite singular noun.

The base noun is:

• en drejskiva = a pottery wheel / potter’s wheel

Definite form:

• drejskivan = the pottery wheel

The ending becomes -an because the noun already ends in -a:

• en skiva → skivan
• en drejskiva → drejskivan

So both nouns in the sentence are definite:

• lerklumpen = the lump of clay
• drejskivan = the pottery wheel

Because Swedish usually puts the definite article on the noun itself.

English:

• the clay lump
• the pottery wheel

Swedish:

• lerklumpen
• drejskivan

So the meaning of the is built into the endings -en and -an.

A separate definite word can appear in Swedish too, but mainly with adjectives:

• den stora lerklumpen = the big lump of clay

Here there is no adjective, so only the ending is needed.

på usually means on, onto, or sometimes in/at, depending on context.

In this sentence, på drejskivan means onto/on the pottery wheel.

It is used because the clay is being placed on the surface of the wheel.

Compare:

• på bordet = on the table
• på stolen = on the chair
• på drejskivan = on the pottery wheel

Swedish prepositions do not always match English one-for-one, but here på works much like English on.

It can mean either one.

Swedish present tense often covers both:

• a general/habitual action: I put
• an action happening now: I am putting

So without extra context, Jag lägger lerklumpen på drejskivan could mean:

• I put the lump of clay on the pottery wheel
• I’m putting the lump of clay on the pottery wheel

If Swedish wants to make the ongoing action extra clear, it can add words like:

• Jag håller på att lägga lerklumpen på drejskivan. = I am in the process of putting the lump of clay on the wheel.

But in normal speech, the simple present is often enough.

Yes. lerklumpen is built from:

• lera = clay
• klump = lump

Together:

• lerklump = clay lump / lump of clay
• lerklumpen = the clay lump / the lump of clay

Swedish forms compound nouns very freely, much more than English often does.

The same is true of drejskivan:

• drej- relates to turning/spinning, especially pottery work
• skiva = disk / plate
• drejskiva = pottery wheel

Learning to spot compounds is very helpful in Swedish.

This is the normal Swedish main-clause order:

subject + verb + object + place

So:

• Jag = subject
• lägger = verb
• lerklumpen = object
• på drejskivan = place/direction phrase

This is very similar to standard English word order:

• I put the lump of clay on the pottery wheel.

Swedish can change the order for emphasis, but then the verb still has to stay in second position:

• På drejskivan lägger jag lerklumpen.

That sounds more marked or emphatic, something like:

• Onto the wheel I put the lump of clay.

Yes, you could, but it sounds a bit more formal or deliberate.

• lägga = the normal everyday verb for put/lay
• placera = place, more formal or precise

So:

• Jag lägger lerklumpen på drejskivan. = natural everyday Swedish
• Jag placerar lerklumpen på drejskivan. = understandable, but less casual

For ordinary physical placement, lägger is the most natural choice here.

In careful speech, jag is pronounced roughly like yahg.

But in everyday spoken Swedish, it is very often reduced to something closer to:

• ja
• or a very weak jag

So Jag lägger lerklumpen på drejskivan may sound closer to:

• Ja lägger lerklumpen på drejskivan

This is normal in spoken Swedish. Learners should recognize it when listening, even if they pronounce jag more carefully themselves.

You normally have to include it.

Swedish usually requires an explicit subject in a full sentence:

• Jag lägger lerklumpen på drejskivan.

You generally cannot just say:

• Lägger lerklumpen på drejskivan

unless it is a special context, such as instructions, notes, or very informal speech.

So unlike Spanish or Italian, Swedish usually does not drop subject pronouns in ordinary sentences.