The tiny words da, po, and si cause students of Esperanto trouble out of proportion to their size. None of them has an exact equivalent in English, and rules for using them can be confusing enough as to require some explanation. Even experienced speakers sometimes misuse them.

Da

Probably easiest is the preposition da, which is simple enough in concept: It’s the preposition we use between a measure of quantity and whatever it is we’re measuring. For example, “a liter of water” is “litro da akvo”. This lets us distinguish between “glaso de akvo” and “glaso da akvo”: The first means a glass with water in it (or a glass used for water) and the second refers to the quantity of water contained in a standard-size glass, whatever that might be. Admittedly, this is rarely an important distinction to make, but sometimes it can be useful — and it’s not all that hard to grasp the idea. Students uncertain whether to use de or da in a phrase may want to imagine a corresponding question that the phrase answers. Is it asking about a quality (de) or a quantity (da)? As PMEG explains:

• Kiom da soldatoj estis? – Estis grupo da soldatoj!
• Kia grupo ĝi estas? – Ĝi estas grupo de soldatoj!

It’s possible to use da after relative or indefinite quantities as well as specific ones — “pli da mono”, “multe da birdoj”, “iom da butero”, and so on. The quantity word shouldn’t be omitted. To say “I bought some butter”, we should use “Mi aĉetis iom da butero” and not just *“Mi aĉetis da butero” as a French speaker might be inclined to phrase it.

While having to distinguish between de and da may be a novel idea for many students, it needn’t be confusing. Some students may be inclined to use the accusative after da, so as with any common error it’s useful to point it out and give them practice in avoiding the mistake.

The real confusion comes from a rule that doesn’t seem to make much sense: We use da only when the object of the preposition refers to something in general (e.g. “faruno”) rather than a specific supply of it (“la/tiu/mia faruno“). So, to ask for a kilogram of flour we’d request “unu kilogramon da faruno”, but if we want a kilogram of some specific stock of flour — say, a bag someone just bought — we instead ask for “unu kilogramon de tiu faruno, kiun vi ĵus aĉetis”. That is, we’d use de rather than da, even though we’re speaking of a quantity of something.

I have to admit that none of the explanations for this odd rule make a great deal of sense to me, but students need to be aware of the rule requiring de in place of da in such cases. Maybe it can serve as an example of how Esperanto isn’t as different from other natural languages as one might have heard.

Po

The preposition po causes problems because it’s so tempting to treat it as an Esperanto translation of the English “per”. Alas, “per” has no equivalent in Esperanto (en or por probably comes closest), and po has no equivalent in English, at least not in terms of propositions.

Po can usually be translated “at the rate of”, so “po 100 kilometroj en horo” means “at the rate of 100 kilometers per hour”. Very often “po __” is equivalent to “__ each” or “__ apiece”. “I gave them 10 dollars each” could be translated as “Mi donis al ili po 10 dolaroj.”

For English speakers, by far the most common mistake is something like *“Ĝi kostas du dolarojn po litro”¹ or *“Ni veturis 60 mejlojn po horo”, but again po is not equivalent to “per”. Those expressions should be “Ĝi kostas po du dolaroj por litro” or “Ni veturis po 60 mejloj en horo”. It’s also common to use an adverb, as in “Ni veturis po 60 mejloj hore”. We can also simply replace po with the accusative, for example “Ĝi kostas du dolarojn por litro”, “Ni veturis 60 mejlojn hore/en horo”.

As a general rule, the object of po is in the nominative case as is usually the case (ha ha) with the objects of prepositions, and students should probably be encouraged to keep to that practice, but it’s worth noting that Zamenhof thought it was not necessarily an error to use the accusative after po. For a discussion of this and the nuances of other words used in expressions of quantity (such as ĉirkaŭ), see PMEG 23.6.

It’s also possible to use po- as a prefix. Popaŝe means “step-by-step”, for example. Sometimes the prefix use can seem pretty close in meaning to the English “per”, as in vendi politre (“to sell by the liter”) or lui ponokte (“to rent by the night”). The entry on po in the Plena Ilustrita Vortaro is worth careful reading.

Si

The reflexive pronoun si has some of the most complicated usage rules in Esperanto, and I’m not going to try to describe them in complete detail here. In brief, however, si is a pronoun that refers back to the subject of the relevant verb, to the whole subject, and to nothing but the subject (as Montagu Butler put it in Step by Step in Esperanto), and it’s used only in the third person.

Si can often be translated as “himself”, “herself”, or “themselves” but that doesn’t mean that the English words are equivalent. For example, “he himself” would be “li mem” in Esperanto, not *“li si” (which doesn’t actually make sense).

Sometimes si can mean “each other” as in “Ili kisis sin”.

Si often removes ambiguity, especially in its possessive form. In English “He saw his friend” can mean that he saw his own friend or that he saw someone else’s. In Esperanto, “Li vidis sian amikon” always means he saw his own friend, and “Li vidis lian amikon” always means he saw someone else’s friend. As that example implies, using si (or sin or sia or sian) is usually not optional. If you can use it, you must.

The “subject” here refers to the simple subject. In the sentence “Karlo kaj Johano alvenis”, Karlo kaj Johano constitute a two-person simple subject, but in “Karlo alvenis kun Johano”, the subject of the verb is just Karlo, even if you rearrange the word order to “Karlo kun Johano alvenis”.

Since si refers to the whole subject, si can’t itself be part of the subject. You can’t for example say *“Karlo kaj sia amiko alvenis hodiaŭ” because the subject is both Karlo and lia amiko, so sia would refer to both of them. On the other hand, “Karlo alvenis hodiaŭ kun sia amiko” is OK, as is “Karlo kun sia amiko alvenis hodiaŭ” because in that sentence the subject is “Karlo”.

To complicate matters, not just verbs but participles as well can have subjects, often implicit ones. Consider the sentence “Releginte sian eseon, Gary decidis sendi ĝin al la redaktoro malgraŭ ĝiaj mankoj.” (“Having re-read his (own) essay, Gary decided to send it to the editor despite its shortcomings.”) Here the participle releginte is an adverb that modifies the main verb of the sentence and has the same subject. Sometimes the subject of a participle is stated explicitly, as in “Ne al glavo sangon soifanta …” and on occasion such a phrase could include the use of si, as perhaps in “Ne al aktoro sin amanta …”.

Exactly how deeply an instructor should get into this clearly depends on the course and the level of the students. Fortunately, the more esoteric examples are almost always clear enough when you hear or read them, and most of us are unlikely to use them in speech or writing, so in practice a basic understanding is sufficient, and if someone does make a mistake, it’s unlikely the mistake will be noticed. As Zamenhof observed in Lingvaj Respondoj, we don’t have to get obsessive about this stuff. (I’m paraphrasing.)

Useful references

The latest revision of La Plena Manlibro de Esperanta Gramatiko (PMEG) can be consulted on the Web. An excellent English-language reference is Montagu C. Butler’s Step by Step in Esperanto, available from the libroservo. Other useful online references are the Reta Vortaro and the Plena Ilustrita Vortaro (PIV). The latter requires you to sign in with your lernu.net account.